Titanate and titania nanostructures and nanostructure assemblies, and methods of making same

ABSTRACT

The invention relates to nanomaterials and assemblies including, a micrometer-scale spherical aggregate comprising: a plurality of one-dimensional nanostructures comprising titanium and oxygen, wherein the one-dimensional nanostructures radiate from a hollow central core thereby forming a spherical aggregate.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/875,786, filed Dec. 18, 2006, which is incorporated herein byreference in its entirety.

This invention was made with support by the U.S. Department of EnergyOffice of Basic Energy Sciences under Contract DE-AC02-98CH10886, andthe National Science Foundation (CAREER award DMR-0348239). TheGovernment has certain rights in this invention.

BACKGROUND OF THE INVENTION

Nanoscale synthesis has traditionally relied on generating nanomaterialsfrom bulk precursors using a number of excellent though imperfectapproaches. For instance, various “top-down” strategies, such asmilling, imprinting, or etching techniques, are limited with respect tothe available geometries, shapes, and sizes of synthesizablenanomaterials that can be efficiently generated. (Mirkin et al., MRSBull. 2001, 26, 506; Xia et al., Adv. Mater. 2003, 15, 353.) Inaddition, diverse “bottom-up” methodologies starting from either atomicor molecular precursors in the gaseous or solution phase often areunable to yield simultaneous control over nanoparticle structure,surface chemistry, monodispersity, crystal structure, and assembly.(Dloczik et al., Nano Lett. 2003, 3, 651; Patzke et al., Angew. Chem.,Int. Ed. 2002, 41, 2446.)

It would be conceptually easier to control the chemical structure ofmatter at the nanometer scale if one were able to start with, transform,and subsequently manipulate nanoscale precursors to obtain the desiredtarget materials. One exciting strategic approach aimed at fulfillingthis objective is associated with the use of localized solid-statechemical transformations via the insertion (Cao et al., J. Solid StateChem. 2004, 177, 2205; Gates et al., J. Am. Chem. Soc. 2001, 123,11500), exchange (Dloczik et al., Nano Lett. 2003, 3, 651; Son et al.,Science 2004, 306, 1009), or deletions (Armstrong et al., Angew. Chem.,Int. Ed. 2004, 43, 2286; Zhu et al., J. Am. Chem. Soc. 2005, 127, 6730)of individual atoms. In other words, existing nanostructures serve asstructural templates from which nanomaterials of a diverse nature and acomplex composition, which may be difficult or otherwise impossible tosynthesize, can be readily generated. Collectively, these types ofreactions with minor alterations could be used to produce manytechnologically important, nanometer-scale crystalline materials, with awide range of size- and shape-tunable properties. (Dloczik et al., NanoLett. 2003, 3, 651; Cao et al., J. Solid State Chem. 2004, 177, 2205;Gates et al., J. Am. Chem. Soc. 2001, 123, 11500; Son et al., Science2004, 306, 1009; Armstrong et al., Angew. Chem., Int. Ed. 2004, 43,2286; Zhu et al., J. Am. Chem. Soc. 2005, 127, 6730; Burda et al., Chem.Rev. 2005, 105, 1025.) The main point involved is that classes of newnanomaterials can be created through reasonably straightforward in situlocalized structural transformations, which are often modifications ofversatile bulk reactions.

As a model system to demonstrate this idea, nanocrystallites of TiO₂(titania) are of great interest for photocatalysts, gas sensors,pigments, and photovoltaic applications, because of their electronic,optoelectronic, and catalytic properties, which are intrinsicallycoupled to their high surface area, porosity, low cost, and chemicalstability. (Hoffmann et al., Chem. Rev. 1995, 95, 69.) Hence, it is notsurprising that groups have been highly motivated to synthesize titanianano-structures by solution chemistry methods, involving either titaniumsulfates, titanium tetrahalides, titanium alkoxides, or otherorganometallic titanium derivatives, under various experimentalconditions, such as the presence of either acidic and alkaline media.(Hoffmann et al., Chem. Rev. 1995, 95, 69; Li et al., J. Am. Chem. Soc.2005, 127, 8659; Zhang et al., Nano Lett. 2001, 1, 81.)

However, there is limited precedence for producing titaniananostructures from an existing nanoscale motif. For instance, TiO₂(B)nanowires have been prepared by heating acid-washed titanate nanowiresat 400° C. for 4 h in air; the titanates in that case were initiallygenerated by adding anatase TiO₂ to a highly concentrated aqueous NaOHsolution. (Armstrong et al., Angew. Chem., Int. Ed. 2004, 43, 2286.)These TiO₂(B) nanowires could be further transformed into their anataseone-dimensional (1-D) analogues as well as into rod-shaped rutile grainsbetween 600 to 800° C. and at 900° C., respectively. (Yoshida et al.,Solid State Chem. 2005, 178, 2179.) Platelike BaTiO₃ and anataseparticles can be synthesized from an H⁺-form of titanate (e.g.,H_(1.07)Ti_(1.73)O₄.nH₂O) with a lepidocrocite-like layered structureusing a hydrothermal soft chemical synthetic process. (Feng et al.,Chem. Mater. 2001, 13, 290.) Titanate nanostructures can be convertedinto their anatase and rutile TiO₂ nanoparticle polymorphs in simplewet-chemical conditions in acidic aqueous dispersions. (Zhu et al., J.Am. Chem. Soc. 2005, 127, 6730.)

A recently reported study from the Alivisatos group aimed to rationallydictate the size and shape of the resultant nanoscale product inselenide nanocrystal systems. (Son et al., Science 2004, 306, 1009;Burda et al., Chem. Rev. 2005, 105, 1025.) However, before the presentinvention, rationally controlling the size and shape of resultanttitania nanoscale products have not yet been reported.

Another relevant area of focus in nanotechnology involves thepreparation of higher-order assemblies, arrays, and superlattices ofvarious, individual nanostructures. The preparation of organizedassemblies of inorganic materials has tended to rely on the use oforganic ligands, additives, or templates. (Whitesides et al. Science2002, 295, 2418; Bartl et al., Acc. Chem. Res. 2005, 38, 263; Park etal., Science 2004, 303, 348; Caruso, F. Adv. Mater. 2001, 13, 11; Colfenet al., Angew. Chem., Int. Ed. 2003, 42, 2350; Sanchez et al., Nat.Mater. 2005, 4, 277.)

However, before the present invention, preparation of higher-orderassemblies of titanate and titania nanostructures have not yet beenprovided, particularly, such preparation has not been achieved withoutthe use of templates.

SUMMARY OF THE INVENTION

The present invention relates to titanate and titania nanostructures,and nanostructure and micrometer-scale assemblies.

In one aspect of the invention, micrometer-scale spherical aggregatesare provided. These aggregates comprise a plurality of one-dimensionalnanostructures comprising titanium and oxygen, wherein theone-dimensional nanostructures radiate from a hollow central corethereby forming a spherical aggregate. Preferably, the aggregateexhibits substantially improved photocatalytic ability.

In one embodiment, the one-dimensional nanostructures of the aggregatescomprise alkali metal hydrogen titanate. Examples of the alkali metalhydrogen titanate include lithium hydrogen titanate, sodium hydrogentitanate, potassium hydrogen titanate, rubidium hydrogen titanate,cesium hydrogen titanate, and combinations thereof.

In another embodiment, the one-dimensional nanostructures of theaggregates comprise hydrogen titanate. The hydrogen titanate preferablyhas an orthorhombic lepidocrocite-type titanate structure.

In a further embodiment, the one-dimensional nanostructures of theaggregates comprise anatase titania.

The one-dimensional nanostructures of the aggregates are preferablynanotubes, nanowires, or a combination thereof.

Preferably, the diameter of the aggregate is about 0.1 μm to about 10μm, or about 0.8 μm to about 1.2 μm. Preferably, the diameter of theinterior core is about 10 nm to about 1 μm, or about 100 nm to about 200nm. Preferably, the average diameter of the one-dimensionalnanostructures is about 5 nm to about 100 nm, or about 5 nm to about 9nm. Preferably, the average length of the one-dimensional nanostructuresis about 10 nm to about 5 μm, or about 100 nm to about 900 nm.

In another aspect of the invention, methods of making micrometer-scalespherical aggregates are provided. These methods comprise mixing analkali metal hydroxide solution, a peroxide solution and a titaniumsource to form a mixture; heating the mixture thereby forming aprecipitate comprising the spherical aggregate, wherein the aggregatecomprises a plurality of one-dimensional alkali metal hydrogen titanatenanostructures, and wherein the one-dimensional nanostructures radiatefrom a hollow central core thereby forming a spherical aggregate.Preferably, the mixture is heated to a temperature of about 50° C. toabout 200° C.

In a preferred embodiment, the alkali metal hydroxide is lithiumhydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide,cesium hydroxide, or combinations thereof.

Preferably, the molarity of the alkali metal hydroxide solution is fromabout 1M to about 10M, wherein the peroxide solution is about 40% toabout 60% peroxide, and wherein the ratio of alkali metal hydroxidesolution:peroxide solution is about 1:1 to about 1000:1, about 25:1 toabout 2:1, or about 10:1 to about 6:1.

Preferably, the titanium source is titanium foil or a liquid suspension(e.g., an aqueous suspension) of metallic titanium powder. An example ofan aqueous suspension comprises about 20 wt % to about 80 wt % metallictitanium powder, wherein the ratio of alkali metal hydroxide:the aqueoussuspension of metallic titanium powder is about 2:1 to about 50:1.Another example is an aqueous suspension comprising about 60 wt % toabout 80 wt % metallic titanium powder, and wherein the ratio of alkalimetal hydroxide:the aqueous suspension of metallic titanium powder isabout 7.5:1 to about 50:1.

In another embodiment, the method further comprises neutralizing theprecipitate thereby forming an aggregate comprising one-dimensionalhydrogen titanate nanostructures.

In a further embodiment, the method further comprises annealing thehydrogen titanate nanostructure aggregate to form an aggregatecomprising one-dimensional anatase titania nanostructures. Preferably,the hydrogen titanate nanostructure aggregate is heated to a temperatureof about 350° C. to about 600° C. during annealing.

In another aspect of the invention, single crystalline anatasenanoparticles are provided, wherein the nanoparticles are at least 95%free of defects and/or dislocations and wherein the nanoparticles hasless than about 0.1% impurities. Examples of the single crystallineanatase nanoparticles are nanocubes or rhombohedra. Preferably, thesingle crystalline anatase nanowire has a diameter of less than about200 nm and the surface of the nanowire is substantially smooth.

In another aspect of the invention, pluralities of anatase nanocrystalaggregates are provided, wherein the nanocrystals are interconnected andaligned onto adjoining wire surfaces with substantially perfectlyparallel lattice fringes.

In a further aspect of the invention, methods of making anatasenanomaterials are provided. The methods comprise (a) mixing titaniapowder and an alkali metal hydroxide solution to form a mixture; (b)heating the mixture thereby forming a precipitate comprisingone-dimensional alkali metal hydrogen titanate nanomaterials, (c)neutralizing the alkali metal hydrogen titanate nanomaterials therebyforming hydrogen titanate nanomaterials; and (d) hydrothermallyprocessing the formed hydrogen titanate nanomaterials thereby forminganatase nanomaterials.

Preferably, the hydrothermal process comprises dispersing the hydrogentitanate nanomaterials in water, and heating at about 100° C. to about200° C.

Preferably, the ratio of the hydrogen titanate nanomaterials:water isabout 2:1 to about 10:3.

Preferably, the temperature to which the mixture is heated in step (b)is about 100° C. to about 145° C., whereby the formed hydrogen titanatenanomaterials are at least 99% nanotubes.

Preferably, the temperature to which the mixture is heated in step (b)is about 150° C. to about 200° C., whereby the formed hydrogen titanatenanomaterials are at least 99% nanowires, wherein nanowires withdiameters less than 200 nm are designated as small, and whereinnanowires with diameters of greater than about 200 nm are designated aslarge.

Preferably, the formed anatase nanomaterials are single crystallineanatase nanowires which are formed from the small hydrogen titanatenanowires.

Preferably, the formed anatase nanomaterials are anatase nanocrystalaggregates which are formed from the large hydrogen titanate nanowires.

The present invention overcomes the shortcomings in the prior art bycontrollably preparing titanate and titania products by simplisticone-pot assembly processes. The methods of the invention also allow forpredictive formation of different size and shapes of the products.Moreover, the as-synthesized crystalline titanate and titania TiO₂products are chemically pure, prepared without the use of eithermineralizers or anionic additives.

Additionally, the methods of making three-dimensional (3D) assemblies ofone-dimensional (1D) titanate and titania nanostructures do not requirethe use of sacrificial templates to render spatial confinement. Suchtemplates tend to yield amorphous or semicrystalline products. Moreover,the methods are easily scaled up to achieve gram quantities of productin a simplistic manner without loss of morphological structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Schematic illustration of the fabrication process for hollowmicrometer-scale spherical assemblies of sodium/potassium hydrogentitanate, hydrogen titanate, and anatase TiO₂ 1D nanostructures,respectively, obtained from either Ti foil or powder upon reaction withH₂O₂ and either NaOH or KOH.

FIG. 2. XRD patterns arising from as-prepared powders of hollowmicrometer-scale spherical assemblies of (a) sodium hydrogen titanate,(b) hydrogen titanate, and (c) anatase TiO₂ 1D nanostructures,respectively.

FIG. 3. Raman spectra of (a) as-prepared powders of hollowmicrometer-scale spherical assemblies of sodium hydrogen titanate 1Dnanostructures, (b) bulk commercial Na₂Ti₃O₇ powder, (c) as-preparedpowders of hollow micrometer-scale spherical assemblies of hydrogentitanate 1D nanostructures, (d) bulk H₂Ti₃O₇ powder derived from acommercial source, (e) as-prepared powders of hollow micrometer-scalespherical assemblies of anatase TiO₂ 1D nanostructures, and (1) bulkcommercial anatase TiO₂ powder, respectively.

FIG. 4. UV-vis spectra of as-prepared powders of hollow micrometer-scalespherical assemblies of 1D nanostructures of (a) sodium hydrogentitanate, (b) hydrogen titanate, and (c) anatase TiO₂, respectively.

FIG. 5. Typical SEM micrographs of as-prepared powders of hollowmicrometer-scale spherical assemblies of 1D nanostructures of (a, b, andc) sodium hydrogen titanate at increasing magnifications. Images ofcorresponding micrometer-scale spherical assemblies of (d) hydrogentitanate and of (e) anatase TiO₂ 1D nanostructures, respectively. (f)EDS data of as-prepared powders of hollow micrometer-scale sphericalassemblies of 1D nanostructures: (1) sodium hydrogen titanate, (2)hydrogen titanate, and (3) anatase TiO₂, respectively. Small Au peaks inthe EDS spectra originate from the gold coating used to eliminatecharging effects prior to SEM imaging, with the C peaks arising from theunderlying conductive carbon tape.

FIG. 6. General cross-sectional TEM images of individual agglomeratesthat were taken from microtomed samples of as-prepared powders of hollowmicrometer-scale spherical assemblies of 1D nanostructures of (a) sodiumhydrogen titanate, (b) hydrogen titanate, and (c) anatase TiO₂,respectively.

FIG. 7. (a) Typical TEM image and (b) HRTEM image of as-prepared 3Dhierarchical, micrometer-scale assemblies of sodium hydrogen titanate 1Dnanostructures, prepared from metallic Ti powder in the presence of NaOHand 50% H₂O₂. The inset of (a) shows the corresponding ED pattern. (c)An enlarged portion of a cluster of 1D nanostructures, as delineated bythe white square in (b). (d) EDS spectra obtained from an individualhollow 3D micrometer-scale spherical assembly of sodium hydrogentitanate 1D nanostructures shown in (a). The Cu peaks originate from theTEM grid.

FIG. 8. (a) Representative low magnification TEM image of a single 3Dhollow micrometer-scale spherical assembly of hydrogen titanate 1Dnanostructures. (b) HRTEM image obtained from the ends of a fewindividual hydrogen titanate 1D nanostructures. (c) and (d) EDS data anda typical SAED pattern, respectively, associated with an individualhollow micrometer-scale spherical assembly of hydrogen titanate 1Dnanostructures shown in (a). The Cu signals originate from the TEM grid.

FIG. 9. (a) A general, low magnification TEM image of an individualmicrometer-scale 3D spherical assembly of TiO₂ 1D nanostructures as wellas the corresponding SAED pattern. (b) HRTEM image taken from the endsof a few TiO₂ 1D nanostructures shown in (a). The inset shows anenlarged portion of a cluster of individual TiO₂ 1D nanostructures(e.g., nanowires in this case), as delineated by the black square. (c)Typical HRTEM image and corresponding SAED pattern of microtomed samplesof hollow micrometer-scale assemblies of TiO₂ 1D nanostructures. (d)HRTEM image obtained from individual microtomed TiO₂ 1D nanostructures.

FIG. 10. SEM images taken from the surface of the titanium foil afterdifferent reaction times: (a) 0 min, (b) 40 min, (c) 1 h, (d) 2 h, (e)and (f) 3 h at varying magnifications, and (g) and (h) 4 h at differentmagnifications, respectively, under hydrothermal conditions at 75° C. inthe presence of 1 M NaOH, 50% H₂O₂, and Ti foil. White arrows in (h)denote localized fracturing of individual sodium hydrogen titanatenanostructure interfaces.

FIG. 11. Time-dependent evolution of the surface roughness (by means ofAFM analysis) of the titanium foil at different growth stages, underhydrothermal conditions of 75° C. in the presence of 1 M NaOH, 50% H₂O₂,and a Ti foil.

FIG. 12. Temporal evolution of titanium concentration, as measured byICP-AES, in solution under hydrothermal conditions at 75° C. in thepresence of 1 M NaOH, 50% H₂O₂, and a Ti foil. The upper blue curvemarked with black arrows represents a hypothesized chronologicalsequence of growth stages.

FIG. 13. XRD patterns from (a) hydrogen titanate nanotubes, (b) hydrogentitanate nanowires, (c) anatase TiO₂ nanoparticles, and (d) anatase TiO₂nanowires, respectively.

FIG. 14. Raman spectra of (a) as-prepared hydrogen titanate nanowires,(b) bulk H₂Ti₃O₇, (c) as-prepared anatase TiO₂ nanowires, and (d)commercially available 5 nm anatase nanoparticles, respectively.

FIG. 15. UV-vis spectra of as-prepared (a) hydrogen titanate nanotubes,(b) hydrogen titanate nanowires, (c) anatase TiO₂ nanoparticles, and (d)anatase TiO₂ nanowires, respectively. The curves are shifted verticallyfor clarity.

FIG. 16. As-prepared hydrogen titanate nanotubes. (a) Low magnificationTEM image. (b) and (c) HRTEM images. The inset of (c) shows an electrondiffraction (ED) pattern. (d) EDS spectrum. The Cu peaks originate fromthe TEM grid.

FIG. 17. As-prepared hydrogen titanate nanowires. (a) Typical SEM image.(b) TEM image. (c) HRTEM image. The inset shows the corresponding SAEDpattern. (d) EDS data. The Cu peaks originate from the TEM grid.

FIG. 18. As-prepared anatase TiO₂ nanoparticles. (a) TEM image. (b) EDpattern. (c) HRTEM image. (d) EDS data. The Cu peaks originate from theTEM grid.

FIG. 19. As-prepared anatase TiO₂ nanowires. (a) General SEM image. (b)

TEM image. An individual anatase nanowire with a diameter of ˜80 nm: (c)TEM image. The inset shows EDS data. The Cu peaks originate from the TEMgrid. (d) HRTEM taken from a portion of the nanowire shown in (c). Theinset depicts the corresponding SAED pattern.

FIG. 20. An individual submicron-sized anatase TiO₂ wire with a diameterof approximately 400 nm: (a) TEM image. The inset shows thecorresponding SAED pattern. (b-d) HRTEM images taken from portions alongthe wire shown in (a), as delineated by individual white squares.

FIG. 21. Schematic representations of crystal structures for theorthorhombic protonic lepidocrocite (H_(x)Ti_(2-x/4)□_(x/4)O₄ (x˜0.7, 0:vacancy)) titanate structure (a and b) and its reconstruction to anataseTiO₂ (c). TiO₆ octahedra are in blue, oxygen atoms are in red, andhydrogen atoms are in yellow. (a) View along the c-axis showingstructural features, associated with the (110) faces of titanate (3×3×1cells). (b) View along the α-axis illustrating structural featurescorresponding to the (011) faces of titanate (1×2×10 cells). (c) Viewalong the b-axis depicting structural features along the (101) faces ofanatase TiO₂ (3×1×3 cells).

FIG. 22. Schematic representation of the size- and shape-dependence ofthe morphological transformation of hydrogen titanate nanostructuresinto their anatase analogues. Step 1 represents the preparation ofhydrogen titanate nanostructures, neutralized from sodium hydrogentitanate nanostructures that, in turn, had been initially hydrothermallysynthesized from commercial anatase TiO₂ powder. Step 2 represents thehydrothermal size- and shape-dependent transformation process oforthorhombic protonic lepidocrocite titanate (H_(x)Ti_(2-x/4)O_(x/4)O₄(x˜0.7, 0: vacancy)) nanostructures into anatase titania nanostructuresat 170° C. for 24 h.

FIG. 23. Photocatalytic activity of the samples in the presence ofProcion Red MX-5B: (a) blank control; (b) commercial anatase TiO₂ (AlfaAesar, 32 nm powder); (c) as-prepared anatase TiO₂ nanoparticles; (d)as-prepared anatase TiO₂ wires.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to nanostructures, such as nanotubes,nanowires and nanoparticles, and assemblies of nanostructures comprisingtitanium, and methods of making same.

Micrometer-Scale Spherical Aggregates Comprising Titanium

The present invention provides micrometer-scale spherical aggregateswhich comprise a plurality of densely packed, individually aligned,one-dimensional (1D) nanostructures comprising oxygen and titanium. Theone-dimensional nanostructures radiate from a hollow central corethereby forming three-dimensional (3D) dendritic assemblies, i.e., thespherical aggregates. These aggregates have a sea-urchin-like shape.

The one-dimensional nanostructures of the aggregates are preferablynanotubes, nanowires, or combinations thereof. Examples of othersuitable one-dimensional nanostructures are nanorods, nanobelts,nanocylinders and nanoparticles aggregates (e.g., individualnanoparticles strung together creating larger linear aggregatestructures).

In one embodiment, the aggregate comprises one-dimensionalnanostructures which consist essentially of alkali metal hydrogentitanate. The alkali metal hydrogen titanate is lithium hydrogentitanate, sodium hydrogen titanate, potassium hydrogen titanate,rubidium hydrogen titanate, cesium hydrogen titanate, or a combinationthereof.

In another embodiment, the aggregate comprises one-dimensionalnanostructures which consist essentially of hydrogen titanate.Preferably, the hydrogen titanate has an orthorhombic lepidocrocite-typetitanate structure. Examples of other preferred structures include amonoclinic H₂Ti₃O₇ structure and an orthorhombic H₂Ti₂O₅.H₂O structure.

In a further embodiment, the aggregate comprises one-dimensionalnanostructures which consist essentially of anatase titania. Anatasetitania can be converted into rutile titania and brookite titania bymethods known in the art.

Typically, the diameter of an aggregate is about 0.1 μm to about 10 μm,more typically, about 0.8 μm to about 1.2 μM, or about 0.5 μm to about1.5 μm. The alkali metal hydrogen titanate aggregates typically have alarger diameter than the hydrogen titanate aggregates which have alarger diameter than the anatase aggregates. Typically, the diameter ofthe central core of an aggregate is about 10 nm to about 1 μm, moretypically, about 100 nm to about 200 nm.

Typically, the average diameter of the one-dimensional nanostructures inan aggregate is about 1 nm to about 100 nm or about 5 nm to about 100nm, more typically, about 1 nm to about 20 nm or about 5 nm to about 9nm. Typically, the average length of the one-dimensional nanostructuresin an aggregate is about 10 nm to about 5 μm or 50 nm to about 5 μm,more typically, about 100 nm to about 900 nm.

Preferably, at least about 95%, at least 97%, or at least 99%, of thespherical aggregates consist of 1D nanostructures. The remaining percentof the aggregates could be attributed to an incomplete growth process.

These aggregates exhibit substantially improved photocatalytic behaviorover current commercial structures. Typically, the aggregates have atleast 30%, at least 40%, or at least 50% improved photocatalyticbehavior vis-à-vis current commercial structures.

In another aspect of the present invention, methods of making themicrometer-scale spherical aggregates described above are provided.

In one embodiment, methods to make the alkali metal hydrogen titanatespherical aggregates are provided. The method comprises mixing an alkalimetal hydroxide solution, an oxidizing agent solution, and a titaniumsource to form a mixture. Preferably, the alkali metal hydroxidesolution and the oxidizing agent solution are mixed together first, andthen the titanium source is subsequently added.

The alkali metal hydroxide is lithium hydroxide, sodium hydroxide,potassium hydroxide, rubidium hydroxide, cesium hydroxide, orcombinations thereof. The molarity of the alkali metal hydroxidesolution is preferably from about 1M to about 10M.

A preferred example of an oxidizing solution is a peroxide solution.Preferably, the peroxide solution is about 40% to about 60% peroxide,more preferably about 50% peroxide.

The ratio of alkali metal hydroxide solution:peroxide solution is about1:1 to about 1000:1, or about 1:100 to about 1000:1. Preferably, theratio of alkali metal hydroxide solution:peroxide solution is about 25:1to about 2:1, or more preferably about 10:1 to about 6:1.

Examples of titanium sources include titanium foil, metallic titaniumpowder and a liquid suspension of metallic titanium powder (e.g., anaqueous suspension). Preferably, the aqueous suspension is about 20 wt %to about 80 wt % metallic titanium powder, more preferably, the aqueoussuspension is about 40 wt % to about 80 wt % metallic titanium powder,or about 60 wt % to about 80 wt % metallic titanium powder.

Preferably, the ratio of alkali metal hydroxide:the aqueous suspensionof metallic titanium powder is about 2:1 to about 50:1, more preferably,the ratio of alkali metal hydroxide:the aqueous suspension of metallictitanium powder is about 5:1 to about 40:1, or about 7.5:1 to about40:1.

The mixture is then heated until a precipitate is formed. For example,the mixture is heated to a temperature of about 50° C. to about 200° C.,or about 60° C. to about 125° C., for about 15 minutes to about 12hours. The heating can take place in a sealed container, for example, anautoclave.

The precipitate comprises the alkali metal hydrogen titanate sphericalaggregates.

In another embodiment of the present invention, the alkali metalhydrogen titanate spherical aggregates are transformed to hydrogentitanate spherical aggregates. In this embodiment, the precipitate isneutralized by methods known in the art. Typically, the precipitate isneutralized by contact with an acidic solution (e.g., hydrochlorideacid, sulfate acid, nitrate acid, or a combination thereof) andsubsequently washed with distilled, deionized water.

In a further embodiment of the present invention, the hydrogen titanatespherical aggregates are transformed into analogous anatase titaniaspherical aggregates. In this embodiment, the hydrogen titanatespherical aggregates are annealed. Annealing comprises heating to atemperature from about 250° C. to about 600° C., or from about 300° C.to about 600° C., more preferably, from about 350° C. to about 500° C.,for about one to about ten hours. Annealing preferably takes place inair.

In another aspect of the invention, the anatase titania sphericalaggregates are converted into rutile titania aggregates or brookitetitania aggregates by methods known in the art.

The initial titanate structural motifs are unaffected by the subsequentchemical transformations. For example, the alkali metal hydrogentitanate aggregates, the hydrogen titanate aggregates, the anatasetitania aggregates, the rutile titania aggregates and the brookiteaggregates are unaffected by the subsequent chemical transformations,i.e., they have essentially the same basic topological morphology astheir initial morphology.

The methods of the present invention are one-pot assembly processes anddo not involve the use of sacrificial templates to render spatialconfinement which tend to yield amorphous or semicrystalline products,and do not require surfactants. (Examples of templates include“track-etch” polymeric membranes; copolymer templates; Teflon membranes;zeolites, and porous alumina or silica membranes (including Anodiscmembranes).)

Single Crystalline Nanomaterials Comprising Titanium Oxide

In another aspect, the present invention provides single crystallineanatase nanomaterials.

In one embodiment, the nanomaterials are single crystalline anatasenanoparticles, including nanocubes and rhombohedra. The sides of thenanocubes typically range from about 5 nm to about 50 nm, from about 8nm to about 16 nm, from about 10 nm to about 14 nm, or from about 12 toabout 13 nm. The rhombohedra of the present invention have a higheraspect ratio than nanocubes. In particular, the widths of therhombohedra are about the same as the sides of the nanocubes. Thelengths of the rhombohedra are about twice as long as their widths.

Examples of other nanoparticles of the invention include spheres,prisms, triangles and tetrapods.

In another embodiment, the nanomaterials are single crystalline anatasenanowires. The average diameters of the nanowires are less than about200 nm, e.g., about 20 nm to about 200 nm. The surface of the nanowiresis substantially smooth.

In a further embodiment, the present invention provides a plurality ofanatase nanocrystal aggregates. In these aggregates, the nanocrystalsare interconnected and aligned onto adjoining wire surfaces withsubstantially perfectly parallel lattice fringes. The average diametersof the aggregates are about 150 to about 500 nm. The average diametersof the nanocrystals are about 4 nm to about 200 nm.

The anatase nanomaterials of the invention are crystalline and solid.Preferably, the nanomaterials are at least 95% free, more preferably atleast 99% free, and most preferably virtually completely free of defectsand/or dislocations. As defined in this specification, defects areirregularities in the crystal lattice (i.e., intrinsic defects). Someexamples of defects include a non-alignment of crystallites, anorientational disorder (e.g., of molecules or ions), vacant sites withthe migrated atom at the surface (Schottky defect), vacant sites with aninterstitial atom (Frenkel defects), point defects, grain boundarydefects, and non-stoichiometry of the crystal. An example of adislocation is a line defect in a crystal lattice.

The anatase nanomaterials of the invention can be converted to brookiteand rutile nanomaterials as known in the art.

Additionally, the nanomaterials are preferably at least 95% free, morepreferably at least 99% free, even more preferably at least 99.9% free,and most preferably virtually completely free of amorphous materialsand/or impurities. Examples of amorphous materials include organicsurfactant molecular groups, such as bis(2-ethylhexyl)sulphosuccinate,undecylic acid, sodium dodecyl sulfate (SDS), Triton X-100, decylamine,or double-hydrophilic block copolymers, which are present on thesurfaces of prior art nanostructures. Examples of impurities include anelement different from the elements of the crystalline structure and avacancy.

In some embodiments of the invention, the outer surface of thenanomaterials contains amorphous carbon, with essentially little or nogroups that are specifically oxygenated, e.g., COOH and OH. The presenceof carbon as well as the absence of the surface oxygenated groups can beverified by IR spectroscopy, X-ray photoelectron spectroscopy or byusing any other surface elemental analysis technique.

In another aspect of the present invention, methods of making the singlecrystalline anatase nanomaterials described above are provided.

The method comprises mixing titania powder and an alkali metal hydroxidesolution to form a mixture. The ratio of the titania powder:alkali metalhydroxide is about 1:25 to about 1:250. The titania powder can be ananatase powder, rutile powder, brookite powder, or combinations thereof.

The alkali metal hydroxide is lithium hydroxide, sodium hydroxide,potassium hydroxide, rubidium hydroxide, cesium hydroxide, orcombinations thereof. The molarity of the alkali metal hydroxidesolution is preferably from about 1M to about 10M.

The mixture is then heated. In one embodiment, designated herein as “thelow temperature embodiment,” the mixture is heated from about 100° C. toabout 145° C., or about 120° C. to about 135° C. In another embodiment,designated herein as “the high temperature embodiment,” the mixture isheated from about 150° C. to about 200° C., or about 170° C. to about190° C. Heating takes place for about 15 minutes to about 12 hours, orfor about a week. The heating preferably takes place in a sealedcontainer, for example, an autoclave.

Upon heating, a precipitate forms. The precipitate comprisesone-dimensional alkali metal hydrogen titanate nanomaterials. Theprecipitate is neutralized using methods known in the art. Typically,the precipitate is neutralized by contacting with an acidic solution,such as 0.1M HCl solution and subsequently washing with distilled,deionized water. The ratio of the alkali metal hydrogen titanatenanomaterials:acidic solution is about 1:10 to about 1:40.

Upon neutralization, the alkali metal hydrogen titanate nanomaterialsare transformed into hydrogen titanate nanomaterials. In the lowtemperature embodiment, the hydrogen titanate nanomaterials are at leastabout 90%, at least about 95%, at least about 99%, or 100% nanotubes. Inthe high temperature embodiment, the hydrogen titanate nanomaterials areat least about 95%, at least about 99%, or 100% nanowires. Nanowireswith diameters less than 200 nm are designated as “small.” Nanowireswith diameters of greater than about 200 nm are designated as “large.”

The hydrogen titanate nanomaterials are then hydrothermally processed toform anatase nanomaterials. The hydrothermal processing comprisesdispersing the hydrogen titanate nanomaterials in water for about 0.5 toabout 5 hours. The ratio of the hydrogen titanate nanomaterials:water isabout 2:1 to about 10:3, or about 2:1 to about 10:1. Then thenanomaterials are heated at about 100° C. to about 200° C. for about 10to about 36 hours. Preferably, heating takes place in a sealedcontainer, such as an autoclave.

Upon hydrothermal processing, hydrogen titanate nanotubes (˜7 to 10 nm)are transformed into high-purity, single-crystalline anatasenanoparticles. Small-diameter hydrogen titanate nanowires (≦200 nm) areconverted into single-crystalline anatase nanowires with relativelysmooth surfaces. Larger-diameter hydrogen titanate nanowires (˜200 to500 nm) are altered into anatase nanowires, resembling clusters ofadjoining anatase nanocrystals with perfectly parallel, orientedfringes.

Thus, while there have been described what are presently believed to bethe preferred embodiments of the present invention, other and furtherembodiments, modifications, and improvements will be known to thoseskilled in the art, and it is intended to include all such furtherembodiments, modifications, and improvements and come within the truescope of the claims as set forth below.

EXAMPLES Micrometer-Scale Spherical Aggregate Comprising Titanium

The examples describe an in situ organization of either sodium orpotassium hydrogen titanate one-dimensional (1D) nanostructures,measuring several hundreds of nanometers in length and up to severalnanometers in diameter, into hollow micrometer-scale sphericalaggregates or sea-urchin-like structures, under a variety of reactionconditions (including temperature variations). These hierarchicalstructures have been produced using a general redox strategy combinedwith a hydrothermal reaction involving a titanium source (e.g., eitherTi foil or Ti powder), a basic NaOH or KOH solution, and an oxidizingH₂O₂ solution. Large quantities of discrete sea-urchin-like structuresof both titanate and titania 1D nanostructures are generated indifferent reaction media, including in solution and on the surfaces ofTi reagent foils and powders.

Three-dimensional (3D) dendritic assemblies of (a) potassium hydrogentitanate, (b) sodium hydrogen titanate, (c) hydrogen titanate, and (d)anatase titania 1D nanostructures are generated in solution and onsurfaces.

The one-pot assembly process does not involve the use of sacrificialtemplates to render spatial confinement which tend to yield amorphous orsemicrystalline products. The initially formed assemblies of alkalimetal hydrogen titanate 1D nano-structures can be subsequentlytransformed into their analogous anatase TiO₂ 1D nanostructures byannealing intermediate hydrogen titanate 1D nanostructures in air.Moreover, the protocol for the controlled synthesis of assemblies of 1Dnanostructures of titanates and anatase can be easily scaled up toachieve gram quantities of product in a simplistic manner, without lossof structure. The detailed time-dependent investigation by scanningelectron microscopy (SEM), inductively coupled plasma atomic emissionspectrometry (ICP-AES), and atomic force microscopy (AFM) of the growthof these nanoscale materials demonstrates that the evolution of theinitial hollow alkali metal hydrogen titanate micrometer-scale spheresinvolves a two-stage process.

Experimental Section

Preparation of Materials. The synthesis and assembly of alkali metalhydrogen titanate 1D nanostructures were performed through a one-stepapproach. In a typical protocol, 16.5 mL of either a 1-10 M NaOH (tosynthesize sodium hydrogen titanate) or KOH (to generate potassiumhydrogen titanate) solution and 1.5 mL of 50% H₂O₂ solution wereinitially mixed into a 23 mL autoclave. Thereafter, either a 1×1 cm²titanium foil or a 0.5-2 mL aqueous suspension of metallic titaniumpowder (<20 μm, 93%, Alfa Aesar, 70 wt %) was subsequently added intothe mixture. The autoclave was then sealed and heated in an oven at60-200° C. for 0-12 h. A ubiquitous white precipitate, that was presentin solution and which also covered the Ti foil surface when used in theexperiment, was observed. Specifically, in the case of the Ti foil, itssurface was repeatedly rinsed with deionized water and dried in air atroom temperature prior to taking SEM images. Separately, the whiteprecipitate collected from solution was then centrifuged and washed withdistilled water until the pH value of the supernatant was close to 7.

The isolated wet precipitate was identified as consisting of hollowmicrometer-scale spheres of either sodium or potassium hydrogen titanate1D nanostructures, which could then be neutralized using approximately15 mL of 0.1 M HCl solution and subsequently washed with distilled,deionized water to yield assemblies of hydrogen titanate 1Dnanostructures. Corresponding assemblies of anatase TiO₂ 1Dnanostructures were synthesized by annealing aggregates of hydrogentitanate 1D nanostructures in air at 350-500° C. for 1-10 h (FIG. 1).

Characterization. As-prepared hollow micrometer-scale spheres oftitanate and anatase 1D nanostructures were initially characterized by anumber of methodologies, including X-ray diffraction (XRD), Ramanspectroscopy, UV-vis spectroscopy, SEM, transmission electron microscopy(TEM), high-resolution transmission electron microscopy (HRTEM),selected area electron diffraction (SAED), and energy-dispersive X-rayspectroscopy (EDS).

XRD. Crystallographic and purity information regarding hollowmicrometer-scale spheres of titanate and TiO₂ 1D nanostructures wereobtained using powder XRD. Diffraction patterns of titanate and anatasestructures were collected using a Scintag diffractometer, operating inthe Bragg configuration using Cu Kα radiation (λ=1.54 Å) from 5 to 60°at a scanning rate of 0.025° and 1° per min, respectively.

Optical Spectroscopy. Raman spectra were acquired with a Ramanmicrospectrometer (Renishaw 1000) using an Ar ion laser (514.5 nm). A50× objective and low laser power density were chosen for irradiation ofthe sample and for signal collection. The laser power was kept lowenough to avoid heating of the samples by optical filtering and/ordefocusing of the laser beam at the sample surface. Spectra werecollected in the range of 1000-50 cm⁻¹ with a resolution of 1 cm⁻¹.UV-vis spectra were obtained for hollow micrometer-scale spheres oftitanate and titania 1D nanostructures, dispersed by sonication indistilled water to obtain homogeneous solutions, at high resolution on aThermospectronics UV1 instrument using quartz cells with a 10-mm pathlength at a resolution of 1 nm. The UV-vis absorption spectra wererecorded using distilled water as a blank.

Electron Microscopy. The size, morphology, and chemical composition ofas-prepared titanate and TiO₂ powder samples were analyzed by electronmicroscopy. For SEM, samples were mounted onto conductive carbon tapes,which were then attached onto the surfaces of SEM brass stubs andthereafter conductively coated with gold by sputtering for 20 s tominimize charging effects. SEM images were taken using a field-emissionSEM (FE-SEM Leo 1550 with EDS capabilities), operating at acceleratingvoltages of 15 kV. Low-magnification TEM and HRTEM images and SAEDpatterns as well as EDS data were obtained on a JEOL 2010F HRTEM(equipped with an Oxford INCA EDS system) at an accelerating voltage of200 kV to further characterize individual 1D nanostructures of titanateand of TiO₂. Specimens for these studies were prepared by depositing anaqueous drop of these samples onto a 300 mesh Cu grid. coated with alacey carbon film. Prior to deposition, solutions containing samples ofmicrometer-scale spherical aggregates of titanate and 1D TiO₂nanostructures were sonicated for 2 min to ensure adequate dispersion insolution.

To further probe the structural nature of the hierarchical structures ingreater detail, micrometer-scale spheres of titanate and TiO₂ 1Dnanostructures were physically microtomed and cross-sectioned for TEManalysis. For the preparation of these microtomed samples, driedproducts were initially dispersed in low-viscosity Spurr's epoxy(Polysciences, Warrington, Pa.), which was then polymerized at 70° C.for 24 h. An Ultracut microtome equipped with a diamond knife wassubsequently used to slice the cured epoxy resin into slivers of ˜70 nmin thickness. These slivers were placed onto carbon-coated Cu grids forTEM observation. TEM images of microtomed samples were taken at anaccelerating voltage of 100 kV on a JEOL JEM1200ex instrument.

Growth Mechanism Studies. The formation process of hollowmicrometer-scale spherical aggregates of titanate 1D nanostructures wasstudied by monitoring (a) the time-dependent morphology and roughnesschange of a titanium foil surface and (b) the temporal progression andalteration in the atomic concentration of titanium in the reactionsolution throughout the growth period of 6 h at 75° C. Because of theexperimental convenience of sampling at lower temperatures, thisspecific reaction temperature was chosen from which to obtainmechanistic insights; the results though are equally valid for theslightly more crystalline products, formed at higher reactiontemperatures (e.g., 130° C.). Similarly, for experimental ease, themorphology change of the surface white precipitate, generated in thepresence of a titanium foil, was analyzed by FESEM. The roughness of theexternal surface of the titanium foil as a function of growth time wasexamined by AFM (Veeco Digital Instruments Multimode Nanoscope Isla,Santa Barbara, Calif.), operated in Tapping Mode using conventionalsilicon tips under ambient conditions. AFM height images were flattenedand plane fitted, prior to roughness analysis. The average roughnessvalue of the samples was extracted from AFM height images (10×10 μm² and2×2 μm²) using commercial DI software. Titanium concentrations of thereaction solution sampled at various time intervals were measured byICP-AES (Perkin-Elmer Optima 3000).

Results

X-ray Analysis. As-prepared solid samples of micrometer-scale aggregatesof titanate and TiO₂ 1D nanostructures were examined by powder XRDmeasurements on a Scintag diffractometer with Cu Ka radiation (FIGS. 2,SI, and S2). The XRD patterns (FIGS. 2 a and S2 c) of an initial,as-prepared alkali metal hydrogen titanate sample obtained after washingwith only water could be attributed to that of either sodium orpotassium hydrogen titanate, depending on the starting reagent. Whereasprior studies (Chen et al., Adv. Mater. 2002, 14, 1208; Sun et al.Chem.-Eur. J. 2003, 9, 2229; and Ma et al., Chem. Phys. Lett. 2003, 380,577) have attributed the identity of these initial solid samples to theformation of H₂Ti₃O₇, the present data suggest that this is not thecase. In FIGS. 2 a and S2 c, it is demonstrated that the XRD patternactually contains peaks at 2θ values of 9.58°, 24.22°, 28.28°, and48.22° which are not consistent with hydrogen titanate. Moreover, theexpected presence of either sodium or potassium in the alkali metaltitanate sample was clearly shown by associated EDS data (FIGS. 5 f andS3 d).

It is noteworthy that the XRD pattern (FIGS. 2 b and S1 b) of the samplesubsequently obtained after neutralization with HCl was different fromthat of the as-prepared alkali metal titanate sample, initially obtainedafter washing with only water. This result implies the presence of astructural transformation from an alkali metal hydrogen titanate to ahydrogen titanate, as either the sodium or potassium ions are replacedby and exchanged with protons during the acid leaching andneutralization reaction step. Moreover, the XRD data, typicallyassociated in the literature with the formation of hydrogen titanate(H₂Ti₃O₇), are not consistent either with the present as-obtainedresults or with data conventionally reported for ion-exchangeabletrititanate nanotubes (Izawa et al., J. Phys. Chem. 1982, 86, 5023). Infact, the XRD pattern for H₂Ti₃O₇ is theoretically predicted to possessa very faint 001 peak (d=0.903 nm) at a 2θ value of 9.795° as comparedwith a very strong 100 peak (d=0.786 nm) at a 2θ value of 11.257°; thepeak intensity of the latter should be approximately five times that ofthe former. The present results clearly show that the relativeintensities among the various peaks do not evince this relativeintensity dependence and are not consistent with the expected XRDpattern for H₂Ti₃O₇.

In fact, a peak in the present data for the hollow micrometer-scalespherical aggregates of hydrogen titanate 1D nanostructures could moreplausibly be ascribed to the 020 peak, observed at d=0.920 nm; thatassignment is consistent with the 020 peak, expected at d=0.926 nm, fora lepidocrocite (H_(x).Ti_(2-x/4)□_(x/4)O₄ (x˜0.7, □: vacancy)) titanatestructure at a 2θ value of ˜9.6°. Indeed, other peaks in the XRD patternof the present hydrogen titanate nanostructures located at 2θ values of24.7°, 28.02°, and 48.22° can be ascribed to the 110, 130, and 200peaks, respectively, of lepidocrocite titanate, as summarized inTable 1. Footnote 1 in Table 1 references Izawa et al., J. Phys. Chem.86:5023 (1982). Footnote 2 in Table 1 references Ma et al., Chem. Phys.Lett. 380:577 (2003). Hence, it is hypothesized that the presentas-prepared assemblies of hydrogen titanate 1D nanostructures actuallymaintain an orthorhombic lepidocrocite-type Ti_(2-x/4)□_(x/4)O₄ (x˜0.7,□: vacancy)) titanate structure (Table 1). (Ma et al., J. Phys. Chem. B2005, 109, 6210; Ma et al., Chem. Phys. Lett. 2003, 380, 577; and Sasakiet al., Chem. Mater. 1997, 9, 602.)

As previously mentioned, the corresponding 3D assemblies of anatase TiO₂1D nanostructures (FIG. 2 c) were synthesized by a dehydration process,involving annealing of assemblies of hydrogen titanate 1D nanostructuresin air at 350-500° C. for 1-10 h. All the diffraction peaks of the netproduct in FIG. 2 c could be indexed to a pure hexagonal anatase phaseof TiO₂, with calculated cell constants of a=b=3.785 Å and c=9.514 Å.The peak intensities and positions of the peaks are in good agreementwith expected literature values (JCPDS File No. 21-1272), as shown inTable 2. Footnote 3 in Table 2 references Bryan et al., J. Am. Chem.Soc. 126:11640 (2004). Indeed, 2θ values of 25.2°, 37.78°, 48.04°,53.92°, and 55.1° could be ascribed to the expected 101, 004, 200, 105,and 211 peaks, respectively, of anatase titania. No peaks of the rutileor brookite phase were detected, indicating the high purity of theproduct.

Moreover, one can clearly observe the presence of an amorphous phase inthe sample of either sodium or potassium hydrogen titanate as well as ofhydrogen titanate, due to the noticeable background in the powder XRDpatterns of these intermediate materials (parts a and b of FIGS. 2 andSI). It is noteworthy that the amorphous phase, however, had almostcompletely crystallized after treatment at 400° C. for 5 h to formanatase TiO₂ (FIG. 2 c).

Raman Analysis. Raman spectra of as-prepared hollow micrometer-scalespheres of 1D titanate nanostructures are shown in parts a and c of FIG.3. Very broad bands are observed near 195, 280, 450, 640, and 920 cm⁻¹.Specifically, the profile in FIG. 3 c shows very similar peak positionsto expected values for protonic lepidocrocite titanate. (Ma et al., J.Phys. Chem. B 2005, 109, 6210 and Sasaki et al., Chem. Mater. 1997, 9,602.) As a means of comparison with the present experimental results,the spectra of commercial bulk Na₂Ti₃O₇ (Aldrich) and of bulk H₂Ti₃O₇(obtained by neutralization of commercial bulk Na₂Ti₃O₇) are shown asspectra in parts b and d of FIG. 3, respectively. It is evident that inthe spectra of these bulk Na₂Ti₃O₇ and H₂Ti₃O₇ samples, there is anabundance of sharp peaks in the lower wavenumber regime located between100 and 400 cm⁻¹ and that there are strong, characteristic peaks near˜850 cm⁻¹ (Sasaki et al., Chem. Mater. 1997, 9, 602 and Bamberger et al.J. Am. Ceram. Soc. 1987, 70, C-48.) These signals are conspicuouslyabsent from the spectra of the present as-prepared hollowmicrometer-scale spherical assemblies of titanate 1D nanostructures.Therefore, it is more reasonable, based on the correlations made inTable 3, to assign the optical modes of the present as-prepared hollowmicrometer-scale spherical assemblies of hydrogen titanate 1Dnanostructures to those associated with protonic lepidocrocite titanate(H_(x).Ti_(2-x/4)□_(x/4)O₄ (x˜0.7, □: vacancy)) sheets. This conclusionis consistent with the XRD data, discussed above (FIGS. 2 b and Sib).The sole caveat is that, because there are few reports directly relatingobserved Raman peaks to specific active modes of layered titanates, theexact assignment of the bands in the Raman spectra may not be fullyaccurate. Finally, the Raman spectrum (FIG. 3 e) of as-prepared hollowmicrometer-scale spherical aggregates of titania 1D nanostructures isassociated with bands at 147, 198, 398, 515, and 640 cm⁻¹, which havebeen assigned to five Raman-active modes of the anatase phase of TiO₂(Table 3), (Sasaki et al, Chem. Mater. 1997, 9, 602; Gao et al., J.Phys. Chem. B 2004, 108, 2868; and Busca et al., J. Chem. Soc., FaradayTrans. 1994, 90, 3181) and correlate well with the signal from the bulkanatase TiO₂ powder (FIG. 3 f). Footnote 4 in Table 3 references Ma etal., J. Phys. Chem. B 109:6210 (2005). Footnote 5 in Table 3 referencesBusca et al., J. Chem. Soc., Faraday Trans., 90:3181 (1994).

UV-vis Analysis. UV-vis spectra of the present as-prepared hollowmicrometer-scale spherical assemblies of titanate and titania 1Dnanostructures are shown in FIG. 4. The sodium hydrogen titanate sampleshows two absorption edges (FIG. 4 a). There is no conclusiveexplanation as to the physical origin of these peaks. Nonetheless, thepresent observations are in agreement with previously reported data fromother groups. Specifically, the absorption peaks from themicrometer-scale spherical assemblies of hydrogen titanate and anatasetitania 1D nanostructures (parts b and c of FIG. 4) suggest that theyare wide band gap semiconductors, a conclusion which is consistent withprevious reports.

SEM Observations. Typical SEM images of aggregates of hollowmicrometer-scale spherical assemblies of titanate and TiO₂ 1Dnanostructures are shown in FIG. 5 in addition to Figures S3 and S4. Onthe basis of these data, it can be observed that the 1D nanostructuresof (a) sodium hydrogen titanate, (b) potassium hydrogen titanate, (c)hydrogen titanate, and (d) anatase TiO₂ self-organize into hollowspherical assemblies, which had never been previously observed for theseclasses of materials. Under the reported synthesis conditions, in termsof morphology, 95% of the samples isolated consisted of 3D hierarchical,micrometer-scale, spherical-like aggregates of densely packed 1Dnanostructures of both titanates and TiO₂, with average diameters forthe assemblies ranging from 0.8 to 1.2 um. The structural features ofthe remaining 5% of samples could be attributed to an incomplete growthprocess. It is interesting to note that the product titania and theseries of intermediate titanate samples retained essentially the samebasic topological morphology, even after intervening chemical processingsteps, such as acid washing and moderate high-temperature annealing.This observation suggests that the initial titanate structural motifs(parts a-c of FIG. 5) observed were unaffected by the subsequentchemical transformations carried out (parts d and e of FIG. 5).

EDS analyses (FIGS. 5 f and S3 d) also showed that the hollowmicrometer-scale spherical assemblies of sodium hydrogen titanate,potassium hydrogen titanate, hydrogen titanate, and anatase TiO₂ 1Dnanostructures were chemically composed of Na/Ti/O, K/Ti/O, Ti/O, andTi/0 elements, respectively, as expected. Neither sodium nor otherextraneous elements were observed either for the orthorhombiclepidocrocite-type hydrogen titanate (H_(x)Ti_(2-x/4)□_(x/4)O₄ (x˜0.7,□: vacancy)) intermediate or for the final anatase titania product.

TEM/HRTEM Observation. To probe the interior of these 3D structures ingreater detail, isolated titanate and titania agglomerates weremicrotomed and analyzed for their cross-sections by means of TEM (FIG.6). It is evident that these 3D assemblies consist of individuallyaligned 1D nanostructures, tightly bundled perpendicularly to a centralvacuous core. In many respects, these 1D nanostructures are analogous tothe spokes of a wheel. In fact, these nanostructures covered the outersurface of the 3D agglomerates to a thickness of 400 nm, while theinterior of these aggregates were hollow with a diameter range of100-200 nm, resembling the microscopic variant of a sea urchin withspines.

Additional higher-resolution TEM data (FIGS. 7-9 and S5) generally showthat the constituent components of the aggregated structures, that is,the 1D nanostructures in question, are single-crystalline in nature withthe presence of surface amorphous layers. Moreover, potassium hydrogentitanate, sodium hydrogen titanate, hydrogen titanate, and anatase TiO₂1D nanostructures consistently possessed a diameter range of 7±2 nm withlengths of up to several hundred nanometers. These results provide forstrong, corroborating evidence that the chemical reaction sequence hadlittle if any impact on the physical dimensions of the 3D structuralmotifs. Furthermore, although these titania and titanate 1Dnanostructures overall appear to be loosely attached to each other,brief sonication for up to 1 h could not visibly disrupt theseassemblies, implying that the interactions among the constituent 1Dnanostructures were particularly strong.

FIGS. 7 a and S5 a show general, low magnification TEM images ofindividual micrometer-sized spherical aggregates of sodium hydrogentitanate and potassium hydrogen titanate 1D nanostructures,respectively. The inset of FIG. 7 a yields an electron diffractionpattern of sodium hydrogen titanate. Parts b and c of FIGS. 7 and S5illustrate HRTEM images recorded from a number of individual sodiumhydrogen titanate and potassium hydrogen titanate 1D nanostructuresemanating from the sea-urchin-like, micrometer-scale assembly, shown inFIGS. 7 a and S5 a, respectively. A 0.36-nm lattice spacing, measuredperpendicularly to the long axis of the 1D nanostructures analyzed(FIGS. 7 c and S5 c), was observed. EDS analysis (FIG. 7 d) also showedthat the micrometer-scale spherical assemblies of sodium hydrogentitanate 1D nanostructures were elementally composed of sodium,titanium, and oxygen, as expected, and that no other elements werenoted.

FIG. 8 a shows the corresponding general, low magnification TEM image ofan individual hollow micrometer-sized spherical assembly of hydrogentitanate 1D nanostructures. On the basis of Raman and XRD data discussedpreviously, this structure is composed of an aggregate of protoniclepidocrocite (H_(x)Ti_(2-x/4)□_(x/4)O₄ (x˜0.7, □: vacancy)) titanate 1Dnanostructures. FIG. 8 b presents a HRTEM image recorded from the endsof a number of single, individual lepidocrocite titanate 1Dnanostructures, originating from the sea-urchin-like, hollowmicrometer-scale assembly, shown in FIG. 8 a. These images suggest thatthe constituent component lepidocrocite titanate 1D nanostructures arecomposed of nanowires and nanotubes. A 0.36-nm lattice spacing betweenthe (110) planes was observed, indicating that the 1D nanostructuressimilarly possess a [110] orientation. EDS analysis (FIG. 8 c) alsorevealed that the hollow micrometer-scale spherical aggregates ofhydrogen titanate 1D nanostructures were elementally composed oftitanium and oxygen, as expected, and that neither sodium nor otherextraneous elements were observed. FIG. 8 d illustrates a SAED pattern,with the two rings observed, noted to be consistent with resultsexpected for the (110) and (200) diffraction planes, respectively, ofthe orthorhombic structure of protonic lepidocrocite titanate(H_(x)Ti_(2-x/4)□_(x/4)O₄ (x˜0.7, □: vacancy)).

Finally, FIG. 9 a shows a general, low magnification TEM image of anindividual hollow micrometer-scale spherical aggregate, composed ofanatase TiO₂ 1D nanostructure products. The inset of FIG. 9 a yields aSAED pattern, with the four rings indexed to the (101), (200), (004),and (211) diffraction planes, respectively, of the hexagonal structureof anatase titania. FIG. 9 b shows a HRTEM image recorded from the endsof a number of single anatase TiO₂ 1D nanostructures derived from thesea-urchin-like assembly, shown in FIG. 9 a. The inset of FIG. 9 billustrates an enlarged section of the HRTEM image, delineated by theblack square. In addition, a 0.352-nm lattice spacing between the (101)planes was observed, indicating that the TiO₂ 1D nanostructures possessa [101] orientation.

FIG. 9 c shows a typical HRTEM image of a microtomed sample obtainedfrom a hollow micrometer-scale spherical aggregate of anatase TiO₂ 1Dnanostructures. Dependent on the orientation of the TiO₂ 1Dnanostructures with respect to the direction of the microtome cut,different lattice planes from discrete TiO₂ nanostructures can beclearly observed. The area delineated by the parallel linear markingshows a lattice spacing between the (101) planes, measuring 0.352 nm.The sector traced out by the solid circle suggests that the individualTiO₂ 1D nanostructures are composed of TiO₂ nanowires with a diameter ofabout 6 nm. The elliptical area at the lower right-hand corner of theimage implies that the TiO₂ nanowire was sliced at an angle. The insetshows a SAED pattern, with three rings observed, indexed to the expected(101), (200), and (004) diffraction planes, respectively, of anatasetitania.

FIG. 9 d illustrates a HRTEM image recorded from the ends of single TiO₂nanowires, indicating that the nanowire grew along the [101] direction.The lattice spacing between the (101) planes (i.e., 0.352 nm) is also inagreement with that of the bulk crystal data. On the basis of thesecollective data, it is seen that individual TiO₂ 1D nanostructuressynthesized using this method are hexagonal and single crystalline, witha structure similar to that of the bulk anatase titania solid. Thisassertion is in agreement with the XRD (FIG. 2 c) and Raman (FIG. 3 e)data, previously discussed, which were taken from a collection of hollow3D micrometer-scale spherical assemblies of aggregated TiO₂ 1Dnanostructures.

Growth Mechanism. There have been a large number of aestheticallypleasing. spontaneously formed examples of crystalline supramolecularassemblies, which have arisen either from the intrinsic packingcharacteristics of molecules or from the interplay of subtleinteractions involving external energy and mass transportconsiderations.

In the present experiment, to investigate the growth mechanism of hollowmicrometer-scale spheres of titanate 1D nanostructures, thecorresponding time-dependent evolution of 3D hierarchical crystalmorphology and of the surface roughness of the foil and of the powdersurface in the presence of NaOH and H₂O₂ were recorded by SEM (FIGS. 10and S6) and AFM (FIG. 11), respectively, over a reaction course of 360min at 75° C. These reaction conditions were chosen due to ease ofsampling. Since effectively identical samples have been obtained attemperatures ranging between 60 and 200° C., it is believed that thegrowth mechanism is temperature-independent. The temporal evolution oftitanium concentration in a 3 mL reaction solution was also monitored byICP-AES over the identical time period (FIG. 12).

Over the time course of the reaction of 360 min, as shown in FIGS. 11and 12, titanium oxidation initially proceeded moderately through anH₂O₂-enhanced oxidation process in NaOH aqueous solution. As thetitanium concentration in solution approached 650 mg/L during theinitial ˜120 min (FIG. 12), the Ti foil surface roughened (FIG. 11) as aresult of a hydrothermally driven direct rolling up of titanate 2Dnanosheets (Figure S7), generated in situ on the Ti foil surface. Thesenanosheets initially emanated from the redox reaction of Ti with H₂O₂ inthe presence of NaOH solution (parts a-c of FIG. 10), then tended tocurl under elevated temperature conditions, and ultimately, transformedinto primary 1D nanostructures of titanates via a nanoscale rollingbehavior (FIGS. 10 d and S7 b). Indeed, it is plausible to assume, basedon the rolling growth mechanism previously reported,⁴² that theinterface between the Ti and the chemically dissimilar peroxide (underhigh pH conditions) provided a unique microenvironment for (a) theinitial formation of a surface sodium titanate hydrogel phase and for(b) the subsequent rolling up of nanosheets,³⁸ formed in situ on the Tifoil, into 1D tubes/wires (Figure S7 b with structures denoted by whitearrows). A narrow window of titanium concentration at approximately thesupersaturation value of ˜700 mg/L was further maintained in thesubsequent growth of these 1D structures (FIG. 12).

During the following time period probed from 120 to 180 min, 1D titanatenanostructures subsequently started to self assemble on the surface,gradually forming 3D spheres, approximately ˜1 μm in size (parts e and fof FIG. 10). The driving force for aggregate formation likely isassociated with factors such as interfacial tension, van der Waalsattractive forces, and the reasonably postulated hydrophilicity ofadjacent titanate surfaces. Another explanation hypothesized for theobserved agglomeration may reside in the slow convection process,creating localized pockets of high concentrations of 1D titanatenanostructures, in the autoclave during the hydro-thermal treatment.Upon further increase of the reaction time from 180 to 240 min andonward to 360 min, the titanate 1D nanostructures continued to organizeon the surface, eventually assembling into fully developed 3D hollowmicrometer-scale spherical architectures, measuring ˜1 μm in diameter(parts g and h of FIG. 10). The observed decrease in titaniumconcentration after 300 min was likely due to a decrease in the amountand availability of H₂O₂, which thereby slowed the overall oxidationprocess (FIG. 12). In addition to the feasibility of this reaction inthe presence of NaOH and Ti foil, regular hollow micrometer-scalespherical assemblies of alkali metal titanate 1D nanostructures couldalso be routinely produced in the presence of KOH and/or Ti powder.

Although not wanting to be bound by a mechanism, it is suggested thatthe observations are the result of a two-stage growth mechanisminvolving the initial formation of primary 1D nanostructures thatsubsequently self-assemble, or more specifically, a“growth-then-assembly” process (FIGS. 10 and 12). It is reasonable thatsubsequent acid leaching and neutralization exchange the Na⁺ with H⁺ toform assemblies of hydrogen titanate 1D nanostructures. Ahigh-temperature treatment and accompanying dehydration processtransform these assemblies of 1D nanostructures into their anatasetitania analogues without atom-by-atom recrystallization of anatase.

In summary, a modified, H₂O-assisted hydrothermal method wassuccessfully developed to synthesize micrometer-scale hollow spheres oftitanate and anatase 1D nanostructures. The template-free andsurfactant-free synthetic methodology of hierarchical structures of 1Dnanostructures is a simple, inexpensive, scalable, and mild syntheticprocess.

Also demonstrated is that micrometer-scale assemblies of TiO₂ 1Dnanostructures are active photocatalysts for the degradation ofsynthetic Procion Red dye under UV light illumination (Figure S8). Thus,the high-quality, micrometer-scale, sea-urchin-like structuressynthesized with their large surface area-to-volume ratio can beincorporated as functional components of a number of devices includingphotonic instruments, dye-sensitized solar cells, as well asphotocatalysts.

Single Crystalline Nanomaterials Comprising Titanium Oxide

In these examples, the size dependence of a number of hydrothermallyprepared titanate nanostructure “reagents” in controllably preparedanatase TiO₂ products by a reasonably mild hydrothermal process coupledwith a dehydration reaction was studied. (Bavykin et al., Mater. Chem.2004, 14, 3370.) Titanate nanowires and nanotubes are converted intoanatase titania nanowires and nanoparticles, respectively, atessentially 100% yield under neutral aqueous, relatively low-temperatureconditions. In fact, the present invention shows that the size and shapeof the precursor titanate structural motif strongly dictate and controlthe eventual morphology of the resulting titania products. Thisinvention allows for localized size- and shape-dependent transformationbetween oxide nanostructure motifs. Moreover, the as-synthesizedcrystalline anatase TiO₂ products are chemically pure, prepared withoutthe use of either mineralizers or anionic additives.

These examples demonstrate that a controllable size- and shape-dependentmorphological change between protonic lepidocrocite titanate and anataseTiO₂ nanostructures can readily occur under relatively simplehydrothermal reaction conditions, in neutral solution, and at reasonablylow temperatures. That is, the size and morphology of the nanosizedreactants can dictate that of the corresponding nanosized products.Thus, the tools are provided for nanoscale design as well as for theprobing of morphology-dependent properties in nanomaterials. Moreover,the TiO₂ nanoparticulate products isolated are single-crystalline, ofthe anatase phase, and of satisfactory purity, without impuritiesarising from brookite or rutile, all of which are desirablecharacteristics for nanostructured materials with potential applicationsin photocatalysis and other chemical processes.

Materials Preparation. A. Titanates. The hydrothermal method initiallydeveloped by Kasuga et al. was employed for the synthesis of titanatenanostructures, which involved a primary reaction between a concentratedNaOH solution and titanium dioxide. (Bavykin et al., Mater. Chem. 2004,14, 3370; Kasuga et al., Adv. Mater. 1999, 11, 1307.) Specifically, acommercial anatase TiO₂ powder (Alfa Aesar, 0.1-1 g) was dispersed in an18 mL aqueous solution of NaOH (5-10 M) and placed into a Teflon-linedautoclave with an 80% filling factor. The autoclave was then oven-heatedat 110-190° C. for 12 h to 1 week. A white precipitate was isolated uponfiltration and washed repeatedly with copious amounts (100 to 200 mL) ofdistilled, deionized water until the pH value of the supernatant hadattained a reading close to 7. After collection by centrifugation andoven drying at 120° C. overnight, the as-produced 1-D sodium hydrogentitanate nanomaterials were neutralized using a 0.1 M HCl solution andsubsequently washed with distilled, deionized water (˜100 to 200 mLuntil the pH of the supernatant had attained a value of (˜7) to preparetheir hydrogen titanate analogues, which were subsequently oven-dried at120° C. overnight.

B. Titania. To synthesize the corresponding anatase nanostructures,dried hydrogen titanate nanostructures (˜50 mg) were dispersed into 16mL of distilled water for 1 h by stirring and then transferred to a 23mL autoclave, which was kept at 170° C. in this case, for 12-36 h. (Fenget al., Chem. Mater. 2001, 13, 290.) A white precipitate was eventuallyrecovered upon centrifugation.

Materials Characterization. All products and intermediate precursors inthis reaction, including hydrogen titanate nanostructures, werecharacterized by a number of different methodologies, including X-raydiffraction (XRD), Raman spectroscopy, ultraviolet-visible (UV-vis)spectroscopy, scanning electron microscopy (SEM), transmission electronmicroscopy (TEM), high-resolution transmission electron microscopy(HRTEM), selected area electron diffraction (SAED), andenergy-dispersive X-ray spectroscopy (EDS).

X-ray Diffraction. Crystallographic and purity information on hydrogentitanate and anatase TiO₂ nanostructures were obtained using powder XRD.To analyze these materials, as-prepared samples of hydrogen titanate andanatase TiO₂, after centrifugation, were subsequently sonicated forabout 1 min and later air-dried upon deposition onto glass slides.Diffraction patterns of these materials were collected using a Scintagdiffractometer, operating in the Bragg configuration using Cu Kαradiation (λ.=1.54 Å) from 5° to 80° at scanning rates of 0.2° per min.

Raman Spectroscopy. Spectra were acquired with a Raman microspectrometer(Renishaw 1000) using an Ar⁺ laser (514.5 nm). A 50× objective and lowlaser power density were chosen for irradiation of bulk hydrogentitanate and commercial anatase TiO₂ samples in addition to all of thenanostructure samples, as well as for signal collection. The laser powerwas kept low enough to avoid heating of the samples by optical filteringand/or defocusing the laser beam at the sample surface. Spectra werecollected in the range of 1000-50 cm⁻¹ with a resolution of 1 cm⁻¹.

UV-visible Spectroscopy. UV-visible spectra were obtained at highresolution on a Thermospectronics UV1 spectrometer using quartz cellswith a 10-mm path length. Spectra were obtained for hydrogen titanateand anatase nanostructures which had been sonicated in distilled waterto yield homogeneous dispersions. UV-visible absorption spectra wererecorded using distilled water as a blank.

Electron Microscopy. The size, morphology, and chemical composition ofsolid powder samples of precursor hydrogen titanate and resultinganatase TiO₂ nanostructures were initially characterized using a fieldemission SEM (Leo FE-SEM 1550 with EDS capabilities) at acceleratingvoltages of 15 kV. Specifically, powders of hydrogen titanate andanatase TiO₂ nanostructures were mounted onto conductive carbon tapes,which were then attached onto the surfaces of SEM brass stubs. Thesesamples were then conductively coated with gold by sputtering for 20 sto minimize charging effects under SEM imaging conditions.

Low magnification TEM images were taken at an accelerating voltage of120 kV on a Philips CM12 instrument, equipped with EDS capabilities.HRTEM images and SAED patterns as well as EDS data were obtained on aJEOL 2010F HRTEM (equipped with an Oxford INCA EDS system as well aswith the potential of performing SAED) at an accelerating voltage of 200kV to further characterize the morphologies of individual nanostructuresof hydrogen titanate and anatase TiO₂. Specimens for the TEM studieswere prepared by depositing a drop of these aqueous suspension samplesonto a 300 mesh Cu grid, coated with a lacey carbon film. Prior todeposition, solutions containing samples of hydrogen titanate andanatase nanostructures were sonicated for 2 min to ensure adequatedispersion in solution.

Elemental Analysis. Inductively coupled plasma mass spectroscopy(ICP-MS) was accomplished using a Perkin-Elmer Sciex Elan 6100 ICP-MSinstrument. A 60-element semiquantitative metals screen was performed onrepresentative samples. The presence of Bi was attributed to impuritiesoriginating from the autoclave.

Photocatalytic Activity. To test the photochemical efficiency ofas-prepared TiO₂ samples, a solution mixture of 100 mg/L Procion RedMX-5B (Aldrich) containing 100 mg/L TiO₂ nanostructures in water wasprepared under irradiation with a UV lamp (maximum emission wavelengthat 365 nm) at a ˜5 cm separation distance. Analogous control experimentswere performed either without TiO₂ (blank) or with commercial nanosizedTiO₂. At given irradiation time intervals, 10 mL aliquots were sampledand centrifuged to remove remnant TiO₂ particles. Supernatant aliquotswere subsequently analyzed by UV-visible spectroscopy at high resolutionusing a Thermospectronics UV1 spectrometer with 10-mm path length quartzcells.

Results

Hydrogen titanates of varied sizes and shapes were controllablyprepared, obtained at different reaction temperatures. Upon subsequentreaction, these nanoscale precursors yielded nanosized titania productswhose structural morphology was intrinsically dependent on the size andshape of the starting reagent nanomaterial.

(i) Hydrogen titanate nanotubes (˜7 to 10 nm in diameter) weretransformed into single-crystalline anatase nanoparticles.

(ii) Small-diameter hydrogen titanate nanowires (≦200 nm) were convertedinto single-crystalline anatase nanowires with relatively smoothsurfaces.

(iii) Large-diameter hydrogen titanate wires (˜200 to 500 nm) werealtered into anatase wires, resembling clusters of adjoining anatasenanocrystals with perfectly parallel, oriented fringes.

XRD. As-prepared solid samples of both hydrogen titanate and anatasenanostructures were examined by powder XRD measurements (FIG. 13). TheXRD patterns of both hydrogen titanate nanotubes and nanowires (FIGS. 13a and b, respectively) do not correspond to either pristine titaniaphases of anatase, rutile, or brookite, or to a mixture thereof. Infact, based on the experimental data, it is more appropriate to ascribethe crystal structure of the instant as-prepared hydrogen titanatenanotubes and nanowires to that of a structural variant of H₂Ti₃O₇. Morespecifically, it is reasonable to assign the XRD patterns to those of anorthorhombic protonic lepidocrocite (H_(x)H_(2-x/4)□_(x/4)O₄ (x˜0.7, □:vacancy)) structure because of the observation of 2θ values at 9.6°,24.7°, 28.02°, 48.22°, and 62°, which can be attributed to the relevant020, 110, 130, 200, and 002 peaks, respectively. (Mao et al., J. Phys.Chem. B 2006, 110, 702; Ma et al., Chem. Phys. Lett. 2003, 380, 577; Maet al., J. Phys. Chem. B 2005, 109, 6210.) Also observed was a strongerintensity of the (020) reflection for nanowires as compared with that ofthe corresponding nanotubes.

All of the diffraction peaks in FIGS. 13 c and d after furtherhydrothermal treatment of the hydrogen titanate nanostructures preparedunder neutral conditions could be indexed to the pure hexagonal anatasephase of TiO₂. The intensities and positions of the observed peaks arein good agreement with literature values (space group 14₁/amd; JCPDSFile No. 21-1272). (Li et al., J. Am. Chem. Soc. 2005, 127, 8659; Zhanget al., Nano Lett. 2001, 1, 81.) No peaks of the rutile or brookitephase were detected, indicating satisfactory purity of the products.

Raman Spectroscopy. Since Raman spectra of both as-prepared hydrogentitanate nanowires and nanotubes were essentially identical, only thespectrum of hydrogen titanate nanowires is presented in FIG. 14 a, whichdisplays very broad bands near 195, 280, 450, 680, and 920 cm⁻¹,respectively. In agreement with the XRD data previously discussed, theobserved spectrum shows very similar peak positions and profiles to thatof protonic lepidocrocite titanate (H_(x)Ti_(2-x/4)□_(x/4)O₄ (x˜0.7, □:vacancy)). (Mao et al., J. Phys. Chem. B 2006, 110, 702; Ma et al., J.Phys. Chem. B 2005, 109, 6210; Sasaki et al., Chem. Mater. 1997, 9,602.) By contrast, the spectrum of a bulk H₂Ti₃O₇ sample, generated bythe neutralization of a bulk Na₂Ti₃O₇ product obtained commercially(Aldrich), is presented as spectrum b in FIG. 14. It is evident thatwith this as-prepared bulk H₂Ti₃O₇ sample, there is an abundance ofsharp peaks in the lower wavenumber regime of 100-400 cm⁻¹ as well as acharacteristically strong peak near ˜850 cm⁻¹ (Mao et al., J. Phys.Chem. B 2006, 110, 702; Ma et al., J. Phys. Chem. B 2005, 109, 6210),all of which are absent in the spectrum of the instant as-prepared 1-Dhydrogen titanate nanostructures. This observation is consistent withascribing the structures of the present invention to the presence ofprotonic lepidocrocite titanate titanate (H_(x)Ti_(2-x/4)□_(x/4)O₄(x˜0.7, □: vacancy)) sheets. The sole caveat is that because there arefew reports directly relating observed Raman peaks to specific activemodes of layered titanates, the exact assignment of the bands in theRaman spectra may not be fully accurate.

The Raman spectrum of TiO₂ nanostructures (FIG. 14) shows the presenceof five characteristic peaks, expected of anatase. No other peaks,corresponding to other titania phases, were observed, indicating thatthe instant as-prepared titanium dioxide nanostructures were likely pureanatase in phase. In fact, observed peaks at 143 cm⁻¹ (E_(g)), 197 cm⁻¹(E_(g)), 399 cm⁻¹ (B_(1g)), 519 cm⁻¹ (B_(2g)), and 639 cm⁻¹ (E_(g))matched well with those of single crystalline anatase (FIG. 14 c).Moreover, the presence of well-resolved, higher-frequency Raman lineswith substantial intensities indicated that the nanostructures werehighly purified with few defects. (Zhao et al., Bull. Korean Chem. Soc.2004, 25, 1341.) FIG. 14 d shows the Raman spectrum of a sample of 5 nmanatase nanoparticles, obtained commercially from Alfa Aesar as acomparison.

UV-visible Spectroscopy. UV-visible spectra of as-prepared protoniclepidocrocite titanate and titania nanostructures are shown in FIG. 15.It is evident that for all the protonic lepidocrocite titanate andtitania nanostructure samples, there is a broad band absorption from 250to 350 nm, due to the transition from the O²⁻ antibonding orbital to thelowest empty orbital of Ti⁴⁺. (Xu et al., Mater. Sci. Eng. B 1999, 63,211.) Moreover, the absorption band of the smaller-sized (tens of nm)protonic titanate nanotubes (FIG. 15 a) is blue shifted relative to thatof the relatively larger (hundreds of nm) protonic titanate nanowires(FIG. 15 b). This blue shift can be rationalized based on a previousstudy, wherein it was found that, with decreasing sample size, theoptical edge tended to shift to higher energy, a phenomenon which wasattributed to quantum size and confinement effects. (Brus, L. E. J.Phys. Chem. 1986, 90, 2555.) The corresponding absorption bands ofanatase nanoparticles and of anatase TiO₂ nanowires are shown in FIGS.15 c and 15 d respectively. Also note that the positions of theabsorption peaks of protonic titanate and anatase titania samplessuggest that these materials are wide band gap semiconductors, aconclusion which is consistent with previous reports. (Zhu et al., J.Am. Chem. Soc. 2005, 127, 6730; Hoffmann et al., Chem. Rev. 1995, 95,69.)

Microscopy of Protonic Titanate Nanostructures. A. Nanotubes. FIGS. 16 aand 16 b indicated that the titanate sample, prepared at 120° C. underhydrothermal conditions, consisted of a large quantity of nanotubes withlengths in the range of several hundred nanometers, outer diameters of˜7-10 nm, and inner diameters of 3-5 nm. HRTEM observations revealedthat these nanotubes normally consisted of three to five layers in termsof wall thickness (FIG. 16 b).

The measured interlayer spacing was found to be about 7.5 Å (FIG. 16 b);the d spacing measurement perpendicular to the tube axis yielded a valueof 3.3 Å (FIG. 16 c). It is well-known that protonic titanates candehydrate during experimental microscopy conditions because of the highvacuum environment and the bombarding effect of electrons, which maycollectively lead to a degree of shrinkage in the interlayer spacing.(Ma et al., Chem. Phys. Lett. 2003, 380, 577.) Thus, the interlayerdistances measured from the HRTEM images herein may not be as accurateand most likely decreased upon loss of hydrated water. This possibilityis consistent with the present observation that the perfect latticetended to degrade after a few seconds of electron beam irradiation.Therefore it was considered reasonable to index the observed 7.5 Ådistance to d₀₂₀ and the measured 3.3 {acute over (Å)} distance to d₁₁₀,respectively, of an orthorhombic protonic lepidocrocite(H_(x)Ti_(2-x/4)□_(x/4)O₄ (x˜0.7, □: vacancy)) structure. In fact, theinset of FIG. 16 c yields an SAED pattern, with the two rings indexed tothe (200) and (110) diffraction planes, respectively, of theorthorhombic lepidocrocite structure. The EDS data (FIG. 16 d) clearlyindicate that the titanate nanotubes are composed of Ti and O, asexpected. No Na was detected in the nanostructures, after washing withHCl. Taking into consideration the likely presence of H in the product,this sample can therefore be attributed to a protonic titanate species,in agreement with XRD and Raman results.

B. Nanowires. FIGS. 17 a and 17 b show representative SEM and TEM imagestaken from the as-synthesized protonic titanate nanowires, neutralizedfrom sodium hydrogen titanate nanowires that had been prepared underhydrothermal conditions at 180° C. In this sample, the as-preparednanowires were measured to be a few microns long and ˜65 to 400 nm wide.Two distinctive populations of diameter distributions of protonictitanate nanowires were observed (FIG. 5I). Though these nanowirestended to aggregate fairly easily, as can be observed from the SEM image(FIG. 17 a), sonication could readily resolve this problem, as shown inthe corresponding TEM image (FIG. 17 b).

The HRTEM image (FIG. 17 c) indicates that, in this particular nanowiresample, the interlayer distance measured along the wire is ˜0.71 nm,whereas the interlayer distance measured perpendicular to the wire is˜0.36 nm. Once again, considering the likelihood of sample dehydrationduring microscopy observations (Ma et al., Chem. Phys. Lett. 2003, 380,577), the 0.71 nm distance could be indexed to d₀₂₀, whereas the 0.36 nmdistance could be assigned to d₁₁₀ of H_(x)Ti_(2-x/4)□_(x/4)O₄ (x˜0.7,□: vacancy). The inset of FIG. 17 c yields an SAED pattern, which can beindexed to the (200) and (110) diffraction planes, respectively, of theorthorhombic lepidocrocite structure, although some displacement of theoriginal spots was noted, indicating that a structural transformationhad taken place upon electron beam exposure. (Ma et al., Chem. Phys.Lett. 2003, 380, 577.) The EDS spectrum (FIG. 17 d) clearly indicatesthat the titanate nanowires are composed solely of Ti and O. As with thetitanate nanotubes, no Na was detected in the nanostructures afterwashing with HCl though the presence of Na was noted by ICP-MS (TableSI). Moreover, taking into consideration the likely presence of H in theproduct, this nanowire sample can therefore be attributed to a protonictitanate species, in agreement with XRD and Raman results.

Microscopy of Titania Nanostructures. A. Nanoparticles. After anadditional hydrothermal cycling involving precursor titanate nanotubesreacted at 170° C. for 24 h, all of the nanosized protonic lepidocrocitetitanate nanotube precursors were transformed into corresponding anataseTiO₂ nanoparticles, which mainly consisted of nanoscale cubes andrhombohedra, as demonstrated by the TEM image in FIG. 18 a Thesenanoparticles have an average size of 12±2 nm. FIG. 18 b shows an SAEDpattern, with the five rings indexed to the (101), (004), (200), (211),and (213) diffraction planes, respectively, of the hexagonal structureof anatase TiO₂, in agreement with that of bulk crystal data. In theHRTEM image of the resulting anatase TiO₂ nanoparticles (FIG. 18 c), onecan clearly observe a 0.352 nm lattice spacing between the (101) planes.The EDS spectrum (FIG. 18 d) shows that these TiO₂ nanoparticles areelementally composed of Ti and O, with the Cu peaks originating from theTEM grid.

Based on these data, the TiO₂ nano-particles synthesized using thismethod are single-crystalline titania, with a hexagonal structuresimilar to that of the bulk crystalline anatase solid. This assertion isin agreement with the XRD pattern and Raman spectrum data (FIGS. 13 cand 14 c), taken from a collection of TiO₂ nanoparticles. It is worthnoting that previous methodologies aimed at synthesizing anatasenanoparticles have been primarily associated with either sol-gel methodsinvolving the use of titanium tetrachloride or titanium alkoxideprecursors or through solution chemistry techniques associated withtitanium sulfates. (Li et al., J. Am. Chem. Soc. 2005, 127, 8659.) Inthese prior reports, such protocols have tended to be associated withthe generation of either chemical impurities or minor impurity phases inthe final anatase TiO₂ products. (Zhang et al., Nano Lett. 2001, 1, 81.)

By contrast, the synthesis of the present invention yields anatase TiO₂nanoparticles with controllable chemical composition (without theobvious presence of impurities) as well as particle morphology, producedunder hydrothermal conditions in neutral aqueous solvent without the useof mineralizers, anions, or similar additives (such as SO₄ ²⁻, NH₄Cl,NaCl, SnCl₄, and so forth). Nonetheless, in some embodiment there can bea significant amount of hydroxyl species on the surfaces of thesenanoparticles, which would be beneficial for an enhanced photocatalyticactivity of these materials.

B. Nanowires. Anatase TiO₂ nanowires were synthesized by a similarhydrothermal soft chemical synthetic method. In this case, protoniclepidocrocite titanate nanowire precursors were used instead. As withthe anatase TiO₂ nanoparticle synthesis, the reaction was also run at170° C. for 24 h. The FESEM image (FIG. 19 a) and TEM image (FIG. 19 b)show that anatase TiO₂ nanowires are formed. The surfaces of the smallerdiameter (≦200 nm) nanowires formed are uneven as compared with those ofthe precursor protonic titanate nanowires, while the surfaces of thelarger diameter (˜200 to 500 nm) wires synthesized are noticeablyrougher than those of the precursor protonic titanate nanowires. Inaddition, clusters of some as-formed nanoparticles also were detected inthis sample.

FIG. 19 c illustrates an individual anatase nanowire with a diameter ofaround 80 nm and with a length of up to a few microns. The EDS data(inset to FIG. 19 c) clearly show that the nanowires are composed of Tiand O elements alone; ICPMS data further confirm the lack of any highconcentrations of impurities (under 0.1%) (Table S1). In the HRTEM image(FIG. 19 d) taken from a portion of the individual anatase nanowireshown in FIG. 19 c, one can observe a 0.352 nm lattice spacing betweenthe (101) planes, indicating that the nanowires have a [101]orientation. The inset to FIG. 19 d shows the SAED pattern, indexed tothe (101) and (002) diffraction planes, respectively, of the hexagonalstructure of anatase. Moreover, the HRTEM images and SAED patterns takenfrom different positions along the nanowire were found to be essentiallyidentical within experimental accuracy, indicating that the entirenanowire is likely to be single-crystalline. In some embodiments, anamorphous coating covers the outer surfaces of some of these nanoscalestructures.

FIG. 20 demonstrates a few representative TEM and HRTEM images of alarger-diameter, individual anatase TiO₂ wire. Its width is around 400nm, and its length can range up to several microns. It is evident thoughthat this wire structure is completely covered by or otherwise composedof aggregates of discrete anatase TiO₂ nanocrystals (FIG. 20 a). Threerepresentative HRTEM images (FIG. 20 b-d, S2-4) taken along the lengthof an individual, larger diameter wire shows that this wirelikestructure is actually composed of a string of adjoining anatase TiO₂nanocrystals. Strong faceting of the nanocrystal building blocks and thepresence of defects at interfaces are clearly observed in these images.These as-formed nanocrystals are interconnected and aligned onto theadjoining wire surface with perfectly parallel lattice fringes, withoutthe apparent presence of misorientations, though these cannot be fullydiscounted. That is, the anatase TiO₂ nanoparticles are all in the sameorientation as the underlying uniaxial wire motif. The spacings of thelattice fringes were found to be about 0.352 and 0.475 nm, respectively,as further shown in Figures S2-4. These two planes could be well indexedas [101] and [002] lattice orientations of the anatase TiO₂ crystal,respectively, according to JCPDS card No. 21-1272. The measured anglebetween these two planes is 68.3°, matching closely with the calculatedvalue based on JCPDS card No. 21-1272 literature data. The orientedarrangement of the anatase TiO₂ nanocrystals was confirmed by SAEDanalysis, which exhibited only one set of diffraction patterns along theentire wire (inset of FIG. 20 a and Figure S5). The same SAED patternshown in the inset of FIG. 20 a was enlarged in Figure S5 and indexed to[101] and [002] planes, consistent with the HRTEM results. The datatherefore suggest that anatase TiO₂ nanocrystals, constituting thewirelike aggregates, are essentially aligned in the same orientation andthat, hence, the as-synthesized anatase TiO₂ wires (including bothas-formed small nanowires as well as nanocrystal aggregates) grow alongthe [101] direction, regardless of the actual wire diameter.

Plausible Formation Mechanism. A. Protonic Lepidocrocite TitanateNanotubes to Titania Nanoparticles. The results suggest that thediameter, i.e., the thickness, of the precursor nanostructure may be acritical determinant factor in governing the resultant shape of theproduct nanomaterial. (Son et al., Science 2004, 306, 1009.) It has beenobserved that, at the nanoscale, there are changes in the reaction freeenergy and the height of the reaction barrier, relative to the bulk.(Van Hove, M. A., J. Phys. Chem. B 2004, 108, 14265; Maradudin et al.,J. Phys. Rev. 1964, 133, A1188.) For instance, by analogy to quantumdots, in the thin precursor protonic titanate nanotubes, the width ofthe reaction zone can become comparable in dimension to the whole widthof the nanotube, due to the relatively small number of atomic layerspresent within a few nanometers of the structure. Hence, the slowpropagation of the reaction front, driven by the gradient of the localchemical potential at or near the reaction zone, may no longer be therate-limiting step of the reaction. Moreover, the thin protonic titanatenanotube may be in a structurally nonequilibrium state and, therefore,merely rearrange into its more stable thermodynamic state. For the thinprotonic titanate nanotubes, a change of morphology to thethermodynamically more stable sphere, cube, or rhombohedron may occur,before the constituent ions have had the opportunity to diffuse,reorganize, and ultimately attain their kinetic equilibrium positions inthe product.

B. Protonic Titanate Nanowires to Anatase Titania Nanowires/NanocrystalAggregates. With lepidocrocite protonic titanate nanowire precursors,which are thicker than nanotubes and which appear to maintain theirnonequilibrium shapes upon reaction, the reaction zone is not asaffected by width considerations of the nanowire itself. Hence,propagation of the reaction front occurs, and the precursornanostructure morphology is retained in the final titania product. (Sonet al., Science 2004, 306, 1009.)

The transformation of larger-diameter hydrogen titanate nanowires (inthis case, between 200 and 500 nm) into structures composed ofaggregates of anatase TiO₂ nanocrystals and of smaller-diameter hydrogentitanate nanowires into single-crystalline anatase TiO₂ nanowires can beexplained by several plausible scenarios. For example, one groupobserved hydrogen titanate nanofibers covered with aggregates of anatasenanocrystals. (Zhu et al., J. Am. Chem. Soc. 2004, 126, 8380.) Toexplain this, it was proposed that the phase transition from titanate toanatase occurred through a topochemical reaction process, in which thehydrogen titanate nanofibers dehydrated due to a reaction with acid,yielding anatase. It was assumed, because of the retention of thenanowire motif, that this dehydration process was accompanied by an insitu phase conversion (albeit incomplete due to formation of a compositestructure), rather than through outright dissolution of titanate andatom-by-atom recrystallization of anatase.

In the present invention, no acid was used. However, note that thelattice mismatch between the (110) plane of protonic lepidocrocitetitanate nanowire substrate and the (101) plane of the anatase TiO₂nanocrystals is very small (˜2%) (Yang et al., J. Am. Chem. Soc. 2005,127, 270); the interplanar distances of d₁₁₀ (3.59 Å, protonic titanate)and d₁₀₁ (3.52 Å, anatase TiO₂) involved are rather similar. Theprotonic lepidocrocite titanate lattice, H_(x)Ti_(2-x/4)□_(x/4)O₄(x˜0.7, □: vacancy), is composed of two-dimensional lepidocrociteγ-(FeOOH)-type sheets in which TiO₆; octahedra are connected to eachother via edge-sharing and protons are localized between the layers. Inother words, the individual lepidocrocite-type host layer resembles acontinuous, planar two-dimensional array. (Ma et al., Chem. Phys. Lett.2003, 380, 577; Sasaki et al., Chem. Mater. 1997, 9, 602.) The layerednature of the protonic titanate structures was clearly observed fromHRTEM images of titanate tubes and wires (FIGS. 16 b and 17 c). Becausethese particular crystallographic features are also common to theanatase TiO₂ lattice and because these lattices are essentiallyperfectly aligned, it is reasonable to postulate that single-crystallineanatase TiO₂ nanocrystals can form and grow in situ from the protonictitanate nanowire surface (FIG. 21). (Zhu et al., J. Am. Chem. Soc.2005, 127, 6730; Yang et al., J. Am. Chem. Soc. 2005, 127, 270.) Thatis, the low interfacial lattice mismatch between titanate and titaniacould lower the heteronucleation energy barrier required for growth ofthe nanoparticles. (Zhang et al., J. Am Chem. Soc. 2005, 127, 13492.)

Hence, while the aggregation of small, independently generated anataseTiO₂ nanoparticles on the surfaces of protonic titanate nanowires is anattractive option, the present observations are better suited to adirect deposition process relying on an in situ nucleation eventfollowed by subsequent oriented crystal growth and precipitation of tinyanatase TiO₂ nanoparticles onto the underlying protonic titanatebackbone. That is, localized dissolution of the precursor protonictitanate nanowires and an in situ transformation into spontaneouslyoriented anatase TiO₂ nanoparticles, undergoing self-aggregation isprobable. This is the basis of the so-called “contact epitaxy”mechanism, previously observed for silver clusters supported on aCu(001) surface. (Yeadon et al., J. M Appl. Phys. Lett. 1998, 73, 3208;Penn et al., Science 1998, 281, 969; Penn et al., Geochim. Cosmochim.Acta 1999, 63, 1549.) In the present case, the driving force for thisspontaneous oriented attachment is the small lattice mismatch betweenthe (110) plane of protonic titanate and (101) plane of anatase TiO₂;the elimination of this pair of high energy surfaces leads to asubstantial reduction in the surface free energy of the resultinginterface, thermodynamically speaking. (Barnard et al., Nano Lett. 2005,5, 1261; Banfield et al., Science 2000, 289, 751.) This effect iscoupled with mechanical relaxation of the highly stressed interface uponepitaxial alignment of the anatase TiO₂ nanocrystals with the underlyingprotonic lepidocrocite titanate nanowire substrate. (Yang et al., J. Am.Chem. Soc. 2005, 127, 270; Zhang et al., J. Am Chem. Soc. 2005, 127,13492; Banfield et al., Science 2000, 289, 751; Liu et al., J. Phys.Chem. B 2004, 108, 2788.) Moreover, this mechanism is conducive toretention of the wire morphology, as the directed self-aggregationprocess of anatase TiO₂ nano-crystals has a low energy requirement forinitially breaking bonds within the hydrogen titanate framework and thenreforming these bonds into anatase titania. (Zhu et al., J. Am. Chem.Soc. 2005, 127, 6730; Liu et al., J. Phys. Chem. B 2004, 108, 2788.)Therefore, this process can take place under moderate conditions of lowtemperature and low pressure, as observed.

Even so, for small precursor protonic lepidocrocite titanate nanowires(≦0.200 nm in diameter), it is relatively easier, based on relativegrowth rates along different planes, to form small equidimensionalparticles similar in dimension to the starting material. Therefore, forthese small hydrogen titanate nanowires, their transformation to anataseTiO₂ nanowires was effectively a simple, in situ phase conversionprocess. Alternatively, anatase TiO₂ nanowire formation can be explainedas resulting from as-formed smaller anatase TiO₂ nanoparticles moreeasily attaching to the hydrogen titanate nanowire surface by attractivevan der Waals forces (the “hit-and-stick” scenario), subsequentlyaggregating, and ultimately fusing to form elongate single crystals.(Pei et al., Langmuir 2004, 20, 7837.)

On the basis of the detailed analysis of the HRTEM and SAED data, theproposed size-dependent shape transformation of hydrogen titanatenanostructure precursors into their anatase TiO₂ counterparts isillustrated in FIG. 22. It is expected that synthetic advances inachieving monodispersity and diameter control over the size and shapedistribution of hydrogen titanate nanostructure precursors can aid inoptimizing the morphological transformation process described herein.

Photocatalytic Activity. The photocatalytic activity of the as-preparedanatase TiO₂ nanostructures of the present invention was evaluated bymeasuring the degradation of synthetic Procion Red MX-5B dye at 538 nmupon photoexcitation with light at 365 nm. It is evident that bothanatase TiO₂ nanoparticles and nanowires (FIGS. 18 and 19,respectively), prepared from nanoscale titanate precursors, are activephotocatalysts, as illustrated in FIG. 23. Moreover, the as-preparedanatase TiO₂ nanoparticles (FIG. 23 c) and wires (FIG. 23 d) exhibithigher photoactivities as compared with similarly sized commercial TiO₂nanoscale powders (FIG. 23 b), from whence the parent titanatenanostructure precursors, used to generate the titania nanostructures ofthe present invention, were initially derived.

The observed enhancement of photocatalytic activity, relative to acommercial sample, may be related to an increase in surface area as wellas with a rise in anatase mass fraction and crystallinity (Xu et al.,Langmuir 2001, 17, 897; Jang et al., J. Nanopart. Res. 2001, 3, 141),characteristic of the pure, as-prepared anatase titania nanostructuresof the present invention. Moreover, an increased amount of hydroxylspecies on the surfaces of as-prepared anatase TiO₂ nanoscale samplescould also explain the heightened decomposition rate observed. (Cao etal., J. Catal. 1999, 188, 48; Vorontsov et al., J. Photochem. Photobiol.A 2001, 144, 193.) In addition, it was also found that the parenttitanate precursor nanostructures exhibited minimal or no catalyticperformance, consistent with a previous report for undoped trititanatenanotube samples (Hados et al., Chem. Phys. Lett. 2004, 399, 512), whichmay be attributed to the higher band gap energy of the titanate samplesrelative to that of titania (FIGS. 15 a and 15 b) and hence a higherenergy of irradiation (than the 365 nm light used in this study) neededfor optimal photocatalytic activation. Furthermore, the anatase TiO₂nanowires (FIG. 23 d) exhibited an increased photoactivity relative tothat of anatase TiO₂ nanoparticles (FIG. 23 c). The morphology-drivenenergy shift and broadening of the absorption spectrum of anatase TiO₂nanowires (FIG. 15 d), with respect to that of anatase TiO₂nanoparticles (FIG. 15 c), can account for the increased photoactivity.(Burda et al., Nano Lett. 2003, 3, 1049.) As implied previously, theparticulate size, degree of aggregation, surface chemistry, and surfacearea of these nanostructures are also important parameters that affectthe data.

Figures designated as S is available free of charge athttp://pubs.acs.org.

TABLE 1 Comparison of Observed XRD Reflections from As-synthesizedHollow Micrometer-scale Spherical Assemblies of Titanate 1DNanostructures (after HCl neutralization) with Literature Data for anH₂Ti₃O₇-type Compound³⁰ (JCPDS File No. 36-0654) as Well as LiteratureValues Reported for Nanotubes of a Known Lepidocrocite TitanateComposition²⁹ nanotubes of as-synthesized 3D assemblies H₂Ti₃O₇lepidocrocite of hydrogen titanate 1D (ref 30, JCPDS No. 36-0654)titanate (ref 29) nanostructures (this work) 2θ (deg) d (Å) I/I_(o) hkl2θ (deg) d (Å) I/I_(o) hkl 2θ (deg) d (Å) I/I_(o) hkl 9.795 9.03  20 0019.5 9.26 100 020 9.6 9.20  64 020 11.257 7.86 100 100 24.5 3.61 ~15 11024.7 3.60 100 110 16.354 5.42  60 101 28 3.17 ~40 130 28.02 3.18  68 13024.372 3.65 100 102 48 1.89 ~50 200 38.76 2.32  19 051 29.75 3.00  60003 62 1.49 ~20 002 48.22 1.88  54 200 36.055 2.49  60 43.93 2.06  60104 48.526 1.876  60 020

TABLE 2 Comparison of Observed XRD Reflections from As-synthesizedHollow Micrometer-scale Spherical Assemblies of Titania 1DNanostructures (after annealing) with Literature Results²⁰ Reported forHexagonal Anatase (JCPDS File No. 21-1272) as-synthesized 3D assembliesTiO₂ (Anatase, ref 20, of anatase titania 1D JCPDS No. 21-1272)nanostructures (this work) 2θ (deg) d (Å) I/I_(o) hkl 2θ (deg) d (Å)I/I_(o) hkl 25.281 3.520 100 101 25.2 3.529 100 101 36.947 2.431 10 10337.801 2.378 20 004 37.78 2.378 72 004 38.576 2.332 10 112 48.050 1.89235 200 48.04 1.892 32 200 53.890 1.699 20 105 53.92 1.698 18 105 55.0621.667 20 211 55.10 1.665 19 211

TABLE 3 Comparison of Observed Raman Peaks (cm⁻¹) with LiteratureData^(14,34) for As-synthesized Hollow 3D Micrometer-scale SphericalAssemblies of Lepidocrocite Titanate and of Anatase Titania 1DNanostructures, Respectively protonic lepidocrocite titanate anatasetitania as-synthesized as-synthesized 3D assemblies of 3D assemblies ofhydrogen titanate anatase titania literature 1D nanostructuresliterature 1D nanostructures band (ref 14) (this work) (ref 34) (thiswork) assignments 145 143 147 E_(g) 195 195 196 198 E_(g) 280 280 396398 B_(lg) 450 450 515 515 A_(lg) 640 640 638 640 E_(g) 920 920

The invention claimed is:
 1. A micrometer-scale spherical aggregatecomprising: a plurality of one-dimensional nanostructures comprisingtitanium and oxygen, wherein the one-dimensional nanostructures radiatefrom a hollow central core thereby forming a spherical aggregate.
 2. Theaggregate of claim 1 wherein the one-dimensional nanostructures comprisealkali metal hydrogen titanate.
 3. The aggregate of claim 2 wherein thealkali metal hydrogen titanate is lithium hydrogen titanate, sodiumhydrogen titanate, potassium hydrogen titanate, rubidium hydrogentitanate, cesium hydrogen titanate, or combinations thereof.
 4. Theaggregate of claim 3 wherein the alkali metal hydrogen titanate ispotassium hydrogen titanate or sodium hydrogen titanate.
 5. Theaggregate of claim 1 wherein the one-dimensional nanostructures comprisehydrogen titanate.
 6. The aggregate of claim 5 wherein hydrogen titanatehas an orthorhombic lepidocrocite-type titanate structure.
 7. Theaggregate of claim 1 wherein the one-dimensional nanostructures compriseanatase titania.
 8. The aggregate of claim 1 wherein the one-dimensionalnanostructures are nanotubes, nanowires, or a combination thereof. 9.The aggregate of claim 1 wherein the diameter of the aggregate is about0.1 μm to about 10 μm.
 10. The aggregate of claim 9 wherein the diameterof the aggregate is about 0.8 μm to about 1.2 μm.
 11. The aggregate ofclaim 1 wherein the diameter of the interior core is about 10 nm toabout 1 μm.
 12. The aggregate of claim 11 wherein the diameter of theinterior core is about 100 nm to about 200 nm.
 13. The aggregate ofclaim 1 wherein the average diameter of the one-dimensionalnanostructures is about 5 nm to about 100 nm.
 14. The aggregate of claim13 wherein the average diameter of the one-dimensional nanostructures isabout 5 nm to about 9 nm.
 15. The aggregate of claim 1 wherein theaverage length of the one-dimensional nanostructures is about 10 nm toabout 5 μm.
 16. The aggregate of claim 15 wherein the average length ofthe one-dimensional nanostructures is about 100 nm to about 900 nm. 17.A method of making a micrometer-scale spherical aggregate comprising:mixing an alkali metal hydroxide solution, a peroxide solution and atitanium source to form a mixture; heating the mixture thereby forming aprecipitate comprising the spherical aggregate, wherein the aggregatecomprises a plurality of one-dimensional alkali metal hydrogen titanatenanostructures, and wherein the one-dimensional nanostructures radiatefrom a hollow central core thereby forming a spherical aggregate. 18.The method of claim 17 wherein the alkali metal hydroxide is lithiumhydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide,cesium hydroxide, or combinations thereof.
 19. The method of claim 18wherein the alkali metal hydroxide is sodium hydroxide or potassiumhydroxide.
 20. The method of claim 18 wherein the alkali metal hydrogentitanate is lithium hydrogen titanate, sodium hydrogen titanate,potassium hydrogen titanate, rubidium hydrogen titanate, cesium hydrogentitanate, or a combination thereof.
 21. The method of claim 20 whereinthe alkali metal hydrogen titanate is sodium hydrogen titanate orpotassium hydrogen titanate.
 22. The method of claim 17 wherein themolarity of the alkali metal hydroxide solution is from about 1M toabout 10M, and wherein the peroxide solution is about 40% to about 60%peroxide, and wherein the ratio of alkali metal hydroxidesolution:peroxide solution is about 1:1 to about 1000:1.
 23. The methodof claim 17 wherein the ratio of alkali metal hydroxidesolution:peroxide solution is about 25:1 to about 2:1.
 24. The method ofclaim 17 wherein the ratio of alkali metal hydroxide solution:peroxidesolution is about 10:1 to about 6:1.
 25. The method of claim 17 whereinthe titanium source is titanium foil or an aqueous suspension ofmetallic titanium powder.
 26. The method of claim 25 wherein the aqueoussuspension is about 20 wt % to about 80 wt % metallic titanium powder,and wherein the ratio of alkali metal hydroxide:the aqueous suspensionof metallic titanium powder is about 2:1 to about 50:1.
 27. The methodof claim 26 wherein the aqueous suspension is about 60 wt % to about 80wt % metallic titanium powder, and wherein the ratio of alkali metalhydroxide:the aqueous suspension of metallic titanium powder is about7.5:1 to about 50:1.
 28. The method of claim 17 wherein the mixture isheated to a temperature of about 50° C. to about 200° C.
 29. The methodof claim 17 further comprising neutralizing the precipitate therebyforming an aggregate comprising one-dimensional hydrogen titanatenanostructures.
 30. The aggregate of claim 29 wherein hydrogen titanatehas an orthorhombic lepidocrocite-type titanate structure.
 31. Themethod of claim 29 further comprising annealing the hydrogen titanatenanostructure aggregate to form an aggregate comprising one-dimensionalanatase titania nanostructures.
 32. The method of claim 31 wherein thehydrogen titanate nanostructure aggregate is heated to a temperature ofabout 350° C. to about 600° C. during annealing.
 33. The method of claim17 wherein the one-dimensional nanostructures comprise nanotubes,nanowires, or a combination of both.
 34. A micrometer-scale sphericalaggregate formed by the method comprising: mixing an alkali metalhydroxide solution, a peroxide solution and a titanium source to form amixture; heating the mixture thereby forming a precipitate comprisingthe spherical aggregate, wherein the aggregate comprises a plurality ofone-dimensional alkali metal hydrogen titanate nanostructures, andwherein the one-dimensional nanostructures radiate from a hollow centralcore thereby forming a spherical aggregate.